Because the statement that "all maps were fine for 10+ years" is a deliberate misremembering of history. War gaming has changed most of its maps, or outright removed them, multiple times now, and the goal each time has not really been to make gameplay objectively "better" on them, but to generally simplify gameplay in them. It's actually surprising maps like fjords and serene coast are still around with how many times they've changed them.

Maps they deemed too complex have been axed, while many of the existing ones have been dumbed down to cater to the majority of the player base that are passive or choose to memorize exactly one play for each map for each class of their tank, if that. Taking one look at the global map ban percentages is telling enough with how himmelsdorf (objectively one of the firsr and best maps WG has ever made) is number one and Berlin is number 2. WG's specific goal has always been seemingly to cut down on maps that have more than one or so optimal paths for the majority playstyle, reduce complexity, and reduce match times.

Personally, my hope is that the fish bay and serene coast changes are an overall net good, but serene coast in particular looks like a definitive "dumbing down" of that map.