Again, I'm not making an argument about the player, because I haven't seen enough. It's just about the principle. If Gyokeres (or any other player) is a world class level talent that fits into the system and the squad, I firmly believe we should not be shying away from him, simply because he is aged 27 years old. It should give us five seasons at the highest level at least.
No-one can know where the team is at the end of that period, but if we actually want to make solid improvements to the squad, we cannot keep tinkering with talents that may or may not develop into something useful. Our forward line is full of those already (Garnacho, Højlund, Amad, etc.). That's the recipe for continuing to languish in that 5th to 8th position in the league.
And I think you are far too concerned with resale value. What do you think the resale value of Højlund, Antony, or Zirkzee would be? Hard to say exactly, but we've definitely lost tens of millions on those already, even though we've bought them young. I'd argue someone like Højlund is one of those that probably should have cost £20-30m like you suggest we go after, but in the end he cost £64m plus add-ons. We aren't going to find these forwards for £20-30m that are anything more than talents to be used as subs and rotation. And the squad desperately needs quality players that can improve us now, instead of in 3-4 seasons. Maybe you've noticed we've only scored 12 goals in 11 games in the league.