I don't think we're a one man team.
That said...
...I get an absolute laugh at Evertonians who say "Yeah, just sell him on for 70M or whatever and replace him."
Two main issues with that line of thinking:
1) The assumption that there is someone out there of his quality we could properly identify without blowing the 70M (or even half of it) on.
2) The assumption that said player would come to us to begin with. If they're good enough to easily replace Lukaku, then they're good enough to go to clubs that are currently in European competition.
Sure I trust Walsh & Koeman to find players that fit into the system and are undervalued and/or undiscovered. But to find a player that could replace Lukaku and what he brings is more than likely not undervalued and not undiscovered. Strikers are the one commodity that aren't not easy to find.
I really really don't want to lose him and want Everton, Moshiri, & Koeman to convince him his long term future lies with the club. Wishful thinking? Yeah, sure it could be. But I don't want to lose arguably the best striker we've had in the last 30 years and replace him with a Beattie, Bent, Johnson, Straq, Jo, etc....
I hate that we missed out on Gabbiadini (hindsight and all that) but I think it would take a top drawer player to replace Lukaku.