You have no idea whether we will win a trophy in the next few years. Yes the PL is highly unlikely but there are cups.
The better club bit doesnt seem to sink in with Celtic fans. Your City's/Utd's/Chelsea's etc wont spend that much money on an unproven kid from the Scottish league, they just wont when they can buy ready made players.
Also as the league is awful, he wont develop much more in the next few years than he has now - you need to be playing with and against better players regularly to do that. So if they wont spend that money now and prices will only increase they wont spend £40-£50 mill in future.
They will see how he goes here after a year or two and if he's still improving and doing well, then they will come along and sign him then. I dont know why your fans find this hard to understand. This maybe why he is happy to come here as its his only route to a top team.
When was the last time a player moved direct from the SPL to top 4 and was a starter?