Well, maybe I've got the wrong impression of him though. He's just never considered a particularly good coach in the Netherlands and not someone who has received a lot of attention for his tactics.
No you're quite correct in my opinion. His most notable tactical feat is probably introducing a defensive 5-3-2 system at Feyenoord in the months before WC 2014, which in turn actually inspired Van Gaal to do the same during the tournament, using a big group of Feyenoord players. Plenty of coaches would love to dominate and play attractive football while dominating possession as well, but there aren't really great examples of teams coached by Koeman that had such a distinctive style. He's always been very pragmatic with his tactics, with mixed results.
I think what people are missing is that Koeman had a bit of a near-death experience recently, being hospitalised with some kind of heart attack a couple of months ago during the start of the pandemic. He's recovered well, but I think this is what has tipped him over the edge because logically it doesn't make
that much sense to take the Barca job now. But he's probably like feck it, this might be my only chance, and technically I guess don't even know for sure what's going to happen with the Euros in combination with the Covid-problems next year - what if it gets cancelled again?- so I can see why he doesn't want to wait until after next summer.
Dutch public will probably react cynically, he pulled the exact same stunt at PSV back in the day when he left mid-season for Valencia, catching 'a train he couldn't miss', according to himself, and he's been mocked for that for years. But there's been much talk about him leaving for Barcelona ever since he started as the manager of the Dutch NT, he even publicly lied about not having an escape option in his contract if Barcelona came for him, media had a field day with that already when it turned out he did in fact have such an option in his contract.
He was doing a pretty good job at the Dutch NT though, personally I felt he was an excellent fit for this group of players, and after four years of pure tragedy and incompetence it wasn't bad to have the guy in charge. I have no idea who we should get to replace him at the Netherlands either.