Are MS Dhoni's powers on the wane? Yes.
Is Dhoni still useful to the team? Yes
The first two games of the three-match ODI series between Australia and India answered both questions above.
In the first ODI, Dhoni walked in to bat at 4 for 3, with his task being to arrest the slide. Over the course of his career, Dhoni has showed that there are few batsmen better than him when asked to play such a role, and this first game was no different. If you have Dhoni in the middle order, it's almost certain that your team won't have a massive collapse. It's not a given that Dhoni will rescue the team and produce a win every time but it's almost certain that he will take the game deep into the innings. In Rohit Sharma's company, Dhoni played the part of rescuer to perfection, but the asking rate skyrocketed in the process. That's when it was clear that his powers have waned. And no, this hypothesis isn't based on a single innings where he couldn't take off but on how his strike rate has fallen over the last few years.
In the second game, Dhoni had Virat Kohli for company. And while Dhoni took time to get going, Kohli shifted gears swiftly to take the burden off his partner's shoulders. That's the essence of good partnerships: sometimes you're the aggressor and sometimes you hope for help from the other end. By the time Kohli was out, Dhoni had found his feet and was ready to choose his prey. He went after Nathan Lyon, while Dinesh Karthik, just like Kohli, eased the burden by playing the dominant role from get go. Run chases are a lot about keeping your composure and breaking a target down into small pieces without taking your eyes off the final goal. Dhoni's ability to do this is priceless. He has been in those situations so many times that nothing fazes him.
This means that even if he isn't the player he used to be, he is still good enough to be a part of the XI. Not to forget his immaculate glovework, accuracy with the DRS, and strategic inputs in the field. Earlier Dhoni the batsman was sufficient to seal the deal; now Dhoni the package is worth sticking with. Also, we must be mindful that while India's top three are the best in the world, there is very little experience and pedigree to follow them if there is no Dhoni in the mix.
Let's try and analyse what might have changed in Dhoni's game from a technical standpoint. Over the years his biggest strengths have been the ability to hit sixes at will, to put the ball in gaps to keep the scoreboard moving, run extremely fast between the wickets, and his understanding of the game to know when he needs to go big. While the last two are still intact, the first couple have declined marginally.
Dhoni's understanding of the situation and its demands is spot on - the mind doesn't get slower with age - and he has kept himself physically fit enough to move swiftly between the wickets. While the ability to hit sixes isn't lost, he does need more time in the middle before executing big hits, and his range of preferred bowlers and deliveries off which to launch big hits has narrowed.
Once your opponents recognise that limitation, self-imposed or otherwise, their fear of getting hit for a six every ball recedes, which in turn allows them to attack more. Fielders inside the circle are at least a few yards closer to Dhoni than they are to Rohit or Kohli. In addition to this, there are five fielders inside the circle in the middle overs, which doesn't allow for easy singles anymore. You need to take a risk to clear the in-field or run the risk of playing a couple of dot balls every over. Taking risks that aren't strictly necessary isn't in Dhoni's DNA. He is hardwired to believe that he can take the team home if he is unbeaten at the end, and so he believes that playing a few more dot balls in the middle overs isn't exactly disastrous for the team.
Batting is all about moving your feet in sync with the ball coming down towards you, and when this movement is slightly out of sync, you cease to play freely. In Dhoni's prime you never got the feeling that he was moving a fraction earlier or late, but these days he is moving earlier and so getting a little late on the ball too.
Previously, he would move with the ball, and that allowed his body weight to go through all the shots - defensive or offensive. These days, it seems that he sometimes plants his front foot a fraction earlier than he used to. This forces him to wait for the ball to arrive, killing the momentum moving into the shot. At other times he waits on the back foot even before gauging the length of the ball.
Dhoni's front-foot stride against spinners was never as long as it has been in the last 12-18 months, and that has been forcing him to manoeuvre the ball into gaps with only his hands - which is not always easy. He has tried a little front-foot press to avoid a long front-foot stride but it is still not inbuilt in his game. He has tried playing the sweep shot and the square cut more often, but his game against spin is built to score in front of the wicket, not square.
He did play with a lot of freedom and allow himself to simply react (and not think about the length of the stride or the trigger movement) during the last edition of the IPL, but that might have something to do with the knowledge that he was going to be playing 14 games in seven weeks, and also the quality (or lack of it) of the bowling attacks he faced.
Since his presence in India's playing XI for the World Cup is a foregone conclusion, it is vital that India find the role he is best suited to playing with the bat. Currently he's slated to bat at five, irrespective of the situation, but it might not be a bad idea to reconsider that going forward. Dhoni's batting style is best suited for arresting a top-order slide and building a platform from which to launch the final assault. So if India lose a couple of early wickets, he should be the man to walk in at four.
There's a theory that Dhoni isn't at his best if the ball is new and the fast bowlers are operating, but I strongly feel that he is too good to be protected. His methods might look ungainly against the moving ball but he has proved time and again that he knows how to handle it as well as anyone else on the circuit. However, if the top three have provided a solid start - say 220 for 2 after 33 overs - you need to be totally unsentimental about Dhoni's spot in the batting order. That's when the likes of Karthik, Hardik Pandya, and even Kedar Jadhav, should be sent ahead of Dhoni.
The hallmark of a champion is to adapt to different roles at different times of his career, and there's no shame in accepting that you can't do things at 35 that you could at 25.