Robles had a disastrous 2020. MAT went to KC for about $1.5 million that off-season. He makes about $3.5 million now.
Taylor hit under .200 with a .253 OBP and 27 strikeouts in 92 at bats in 2020 and was arb-eligible again after making a little over $3 million (prorated down to a little over $1 million for the shortened season) - again, that's for hitting under .200. Robles was one year removed from 2019, in which he'd put up 4.4 WAR (mostly defense, yes) as a 22-year-old.
If the Nats had tendered Taylor a contract, he'd probably have gotten a further raise, because that's how arbitration works. ~$3.75-4 million is a lot for a backup outfielder who can't hit his weight, especially to back up a much younger and cheaper guy who either can or can't hit and does that from the same side. Back then, people thought the real Robles was 2019, and 2020 was a COVID fluke.
And remember, the market showed that what the Nats would have paid MAT was way more than he was worth. Yes, they could have non tendered him and then tried to re-sign him for less, but one could hardly blame MAT for looking elsewhere. I'm glad it's worked out for him; he seems like a good guy.