Your entire argument rests on unfair expectations. They didn't get a cure for cancer for their 3.5M either...so "Boo!" Chris Archer I guess.
They paid for a depth starter. (Hell, half of that going rate really, but I'm being as fair as possible to your argument) They've gotten that and one who has pitched really freaking well AND stayed healthy. I mean, if they paid him like Noah Syndegaard then sure. They paid what you pay your 5th guy in your bullpen on an arbitration deal much less a starter. Their ROI is through the freaking roof.
Who has Archer stolen his starting spot from? Bundy? Sands? Oh right.....nobody. They've desperately needed him and he's been there every fifth day. I'd argue even if his ERA was north of 5, just by being a warm, competent body was worth the 3.5M, much less the production.