In the past when players reached a certain level they left, because other clubs offered a better, indeed much better, place to continue move to a higher level. Young, Barry, Milner, and so on. Villa was struggling to maintain whatever position it was at. We were not a big club, and hadn't been since WWII. Villa would have benefitted from those players staying, but not to the extent that it would have put us in the big club league.
Bit different now. Jack staying is a real option for him because the owners have the ambition, means and expertise to put the club back at the top level. There are no certainties in this world. The club might not make it, or might take longer than hoped for. On the other hand, if Jack moves he becomes just one cog in a big machine and there's no guarantees that he'll fit. Whatever, there are good reasons for him to develop his career with us, and my guess is that he'll do that for at least another year.
Another factor is his leg. We're probably a good place for him to see if he can get that trouble behind him.