Struggling to understand your logic other than you clearly do not like the player (though, after that disgusting instagram post, can't say I blame you).
Miles better at his age... Adel Taarabt was miles (and miles) better than 80% of current attackers in the premiership at 19. Fat good that was. Who cares how many goals he scored at 21. Some players develop later than others. Incidentally Richarlison actually scored 4 league goals last season, all in purple patch which then dissipated as quickly as it came, to the extent that he was benched by a mid/lower table team. And you would prefer him to a player who kept a doomed team in the Premiership practically on his own.
This "better than..." is - no disrespect - a puerile argument that should remain in school playgrounds. Better in what? Is he better than Dele in getting into the box and scoring goals? No! Is he better than Eriksen in seeing space and threading an impossible ball into it? No, again. But is he better at taking on opponents, a beating one, two or even three players in areas which hurt the opponent and disrupting their shape. Most definitely YES; and that also includes Moura and Lamela, who have different strengths to bring to the table. So the question is not whether he is 70 mill better but, IMHO, should be rephrased as... is it worthwhile spending 70 mill to bring this skill set into our squad? Those supporting a Zaha transfer - myself included - will say Yes because if we fail to make top 4 next season, it will almost certainly be because we have failed to beat enough teams beneath us, teams who set out to frustrate putting everyone behind the ball. While we need to see what Moura can contribute in these situations, the impact of the rest of the current squad is well known and we will struggle again to break these teams down without someone able to pull out something special in terms of flash of magic. For that, I would definitely pay 70 mill.
As for being a "good option off the bench"... I have seen enough supposedly learned posters claim that player X or Y is not good enough for our first team (just go to the beginning of the Harry thread) only for them to have to eat big chunks of humble pie months later. All I can say is.... does Zaha have the attributes to become a regular starter? Once again, anyone without an underlying bias, would have to conclude that the potential is there. Whether he then takes that up, is something only time would tell.
Ok. There's a lot here. Before I respond to some of your points, here's a quick, clear breakdown my overall points so you can "understand my logic."
a) Wilfried Zaha is not worth 70 million pounds (and it has nothing to do with IG). He's never produced like a 70 million pound player, and at 25, he's extremely unlikely to take the exponential leap to become that kind of elite attacker.
b) Signing him would take up a huge amount of our budget and could be better used in other places.
c) I would also not be in favor of spending 50 million pounds on Richarlison. I was only using him as a point of comparison and to illustrate value.
Now to your points.
"Adel Taarabt was miles (and miles) better than 80% of current attackers in the premiership at 19."
This one was my favorite. When Adel was 22 years old, he had made precisely 10 first team starts, scoring precisely zero goals. Miles (and miles) better than 80% of young attackers in the Prem, hahaha ok.
"Incidentally Richarlison actually scored 4 league goals last season, all in purple patch which then dissipated as quickly as it came, to the extent that he was benched by a mid/lower table team"
Richarlison appeared in all 38 Prem games for Watford and started 32, so not sure when he was benched. Also, his .34 xG per 90 is an extremely high number for a winger, better than Sane and Zaha for comparison. In truth, he was actually unlucky with his finishing, which hints that he's capable of getting much better.
"This "better than..." is - no disrespect - a puerile argument that should remain in school playgrounds. Better in what? Is he better than Dele in getting into the box and scoring goals? No! Is he better than Eriksen in seeing space and threading an impossible ball into it? No, again. But is he better at taking on opponents, a beating one, two or even three players in areas which hurt the opponent and disrupting their shape. Most definitely YES"
Besides the hilarious irony that you opened this rebuttal by talking about how Taarabt is "better" than 80% of Prem players at 19, better means more productive. Pretty easy to understand. My point is that, our attack would be more productive with Kane, Son, Eriksen, and Alli as our attacking four than Zaha in any of those roles. If a club like Spurs are going to spend 70 million on an attacker, you would think he'd be able to get into the first XI. He's not more productive than any of the aforementioned 4, and Lamela and Moura actually have better/similar numbers, even though last year was a much smaller sample size so those numbers are a bit skewed.
"is it worthwhile spending 70 mill to bring this skill set into our squad?"
Which brings us to our main point of contention. Is it worth 70 million pounds to add a peak age bench option who will help us beat "lower teams" (who we almost always beat anyway)? You're entitled to your opinion, but it's a hard 'no' from me. Spending 70 mil on a 25 year-old who has the "potential" to break into our first team is not a logical use of money. But you're entitled to your own.