I think with Mahrez you keep him as long as you can, you can't replace him. leicester will still get a big fee next Jan if you really want to piss him off. I think £100m+ is not unreasonable and would likely be more achievable in summer.
He won them the title alright. I think you never really allow the best players to leave, they have release clauses or run down their contract so you're forced to take a price. Either that or start getting pictured with bongs and prostitutes or piss everyone off sufficiently to seek an exit. Things can change in 6 months and even if he sulks for a bit, he'll want to play at some point, between windows you have fresh if diminishing chance to renegotiate new contract, maybe give a release clause, 3/4 of the year is outside a transfer window.
Plenty of players have downed tools only to pick them back up not long after. And plenty of those have gone on to sign new contracts with existing club, it sometimes just needs more hope, new manager and a much bigger pile of money. Only clubs get to interact or negotiate directly with them on regular basis. Lukaku gave Koeman a chance to move us on and even when he hadn't, he still nearly signed. I think in January you're always going to get some very good players feeling the season is over, so you tend to get more strops.