IMO Kucherov has absolutely cemented himself as part of the nucleus and it's he and Hedman who are the two set in stone "keep at all cost" players. As amazing as Bishop is he more expendable than Kuch as there's a better chance that Vasy becomes an elite goalie (or at least one good enough to win a Cup with) than there is that we can get another forward of Kucherov's caliber. Losing Bishop might set us back a year or two before Vasy hits his stride but losing Kucherov would kill us offensively for years to come. Obviously we want to hang onto Bishop if it's practical to do so, but if somebody offers him an absurd contract at some point we would be better off going with Vasy and using the cap savings to improve in other areas so that we don't need a top 3 goalie carrying the team to be competitive.
The problem is that unless we're willing to give up on Vasy this summer and accept that he will be exposed and potentially lost to the expansion draft next year, we can't resign Bishop before he hits free agency and therefore cannot know for certain what his cost will be when it comes to making a decision on Stamkos. Maybe Yzerman decides that Bishop is too important to risk losing and that he'd rather give up on Vasy now and worry about finding Bishop's eventual replacement down the road, but if he's going to try to keep both goalies then the best he (and we) can do is guess at what it would take to resign Bish and base his/our decision on that (well technically Yzerman could present his strategy to Bishop and discuss how much he'll want, but it wouldn't be binding so it's really just a more informed guess than we can make.) The risk of course is that we let Stamkos walk now and then Bishop leaves the following year, but we would at least get the consolation prize of having a ton of cap space to work with so it wouldn't be all bad.
Assuming that we are going to try to keep Bishop and either extend him now or designate a reasonable amount for his raise next summer, that leaves Stamkos versus the likes of Johnson, Palat, Drouin, and Killorn. Johnson has been better in the playoffs and the injury concerns between the two offset at this point, and I would argue that Drouin will be better than Stamkos going forward. That leaves us finally with one-dimensional goal scorer Stamkos against multi-dimensional 200 foot players Palat and Killorn, and I would go with the depth, flexibility, and lower risk option myself. So Stamkos goes.
This is all assuming that Carle is bought out and Filppula is traded, but that we keep Callahan (because his contract is immovable) and Garrison (because he's too valuable to our defense.) If we could somehow move Cally without retaining too much then we might have more options, but otherwise I think we're looking at losing Stammer in order to retain everybody else of importance.