What we paid for him is utterly irrelevant. Our ambition given the money we've spent over years now should be to break into the top 6. Every player's ability/performance should be judged against that, not what we paid for them. He's either good enough for that, or he isn't. If he isn't performing at the required level, he is a problem and gets criticism, which should go for everyone. It's as simple and as fair as that. If we're not setting the top 6 as the minimum aim in the short-term, given our spending, we may as well not bother.
The "personal attack" tactic used to discourage criticism of a player's ability is unfortunately as predictable as it is boring. Why would anyone blame a player for being picked? Of course the club are to blame if they sign players that can't produce what the club's ambitions required. People generally don't blame any player for not being good enough as long as they're genuinely trying, which 99% of the time they are.
The fact that he's been a first choice striker has cost us a huge amount over the last few years and he only recently started paying back on that. Davies never did. If the last 4/5 years are the result of playing the "long game", then that tactic has been a disaster. If DCL does become a £50m striker, fantastic. But it won't erase the 3 years before that, where he was horrendous. I'd much rather he'd been out on loan developing while we had a competent striker in place. We wouldn't want to be hanging our hat on Branthwaite or Nkounkou as first choice for the same reason. There's every reason to expect they'd be nowhere near the level we need.