I don't think this has anything at all to do with tennis players seeming to be leaving Nike... For a start, the divisions of Nike pull their weight or they would be reigned in after a short time - possibly less than 2 years. Nike wouldn't stand for, very long at least, propping up a division that had to be paid for by the others.
Secondly, we have no idea whatsoever when the contracts of the players leaving Nike were up. They're not in perpetuity. Del Potro for example may have been offered a three year deal shortly after winning the 2009 US Open - which would be up now.
Likewise, the tennis clothing market especially seems to be maturing again after a relatively long haitus where only two or three brands really succeeded and the others struggled. The fact we're seeing Wilson clothing quickly become a relatively common sight again suggests it's companies like them and Head who're branching out and approaching players with offers moreso than Nike ditching players. These companies often have existing direct contacts courtesy of their racquet contracts/arrangements so the likes of Wawrinka, Berdych, Dimitrov or whoever moving over to their clothing division is not remotely surprising to me.
Similarly, now that Federer is closer to the end of his career and Nadal has been relatively unsuccessful in terms of taking over those reigns in the last 18 months or more (either in his results or as the person to take over the head "face of tennis" role post-Federer *) Nike is perhaps just taking a broader view for a period and being pragmatic about who they sign. Most of the players they've endorsed haven't broken through to the top echelon and most of them will be on two or one year arrangements of pretty moderate value, so other companies will be stepping in and making offers for sure. Maybe Nike is just reassessing the field - taking a closer look at guys like Janowicz, Tomic etc.
Also worth noting, the total cost of the players who're leaving Nike would be less money than Nike spends on Federer promo events per year. His and Nadal's total endorsement costs combined would surpass everyone else put together by miles (I'm talking just about the men here). Losing Del Potro or Dimotriv or Berduch or all of them would be less about saving money than it would their not being bothered to get into a bidding war with another company for players who've had years to break through and haven't really yet.
(*a local shop I go to doesn't even stock Nadal's clothing anymore. They still have plenty of his last season's gear available in the sale bins and the guy there told me no-one was buying it. Federer's gear flies out the door still though.)