If you made that argument about Bale then you might have a point. For most players, it's a laughable case to make though.
Its really not, most players will move on eventually and most players get replaced, this year, or 4 years from now, change happens, who cares.
The difference is 35mil is massively overpriced, a great deal.
I wouldn't sell Wilshire, because we wouldn't get 50mil for him.
Pay attention to whats going on Modric has already said he wants to leave, that means chances of signing a new contract are slim, meaning 3 or 4 years from now, every year he'll be worth less then leave on a free. At this stage 35mil is great, if he breaks his leg and missed 18 months, he'll be worth maybe, luckily 10mil in his final year of his contract based off no one knowing if he was back to his best.
Wilshire, if he breaks his leg next season, still intends to stay at Arsenal, has not said he wants to leave, wouldn't fetch an offer of 35mil and will sign new contracts in the future which will keep his value essentially up to date.
Completely different situations so try to keep up with whats going on with Modric, wants to leave, will decrease in value every single year, 35mil is an awesome price for a guy who wants to leave. Its good business, nothing more or less.