I don't see the sense in even considering letting Cork go unless there is someone coming in who is a major step up in quality. If Saints have pretentions to Europe, be it Europa or CL, then the squad needs to have strength in depth. That may mean the kids have to take a wee side-step in adjustment. Jack is ahead of what we have coming through and he is still isn't the finished article. He has got markedly better in the Premier League seasons so far. There's no reason to think he won't get better still. Two seasons ago he wouldn't have been good enough for this team. Now he is an asset.
1) If Saints want to make some noise in the European Cup, they will probably have to get better before they get deeper.
2) There's plenty of reasons to think Cork won't get better. He's 25 and has a lot of experience. Odds are he is at or very near peak. It's not at all impossible that he takes a step up, but it's foolish to expect it.
3) What if we don't make Europe? What if Cork doesn't want to be a guy who predominantly rides the bench and only gets trotted out for Cup Matches?
4) What if there is in fact someone better? The club has a pretty good record with transfers. If Lovren, Lallana, and Lambert can all be replaced with players equal or better under less-than-optimal circumstances then why can't Cork be replaced?
Stop hoarding players, y'all. It's stupid, and it's a big part of the reason why some of the big clubs are having issues. If you sign Cork, you should be prepared to commit to him as a starter for at least two years. If you can't do that, then perhaps it is better for both sides to part ways.
I like Cork, but seriously we can't re-sign all of our players AND bring in better players AND bring through the youth. The latter two are critical to our success. It wasn't our choice this summer but if Lallana, Lovren, Shaw, and Chambers don't go then we wouldn't have Bertrand, Pelle, Tadic, Mane, or Alderweireld. It's amazing to me that people here simultaneously have ambitions for Saints to be some kind of European power and at the same time are too wussy to ever let go of players other than the obvious ones whose exits are meaningless considering they never play anyway.