My personal suss on it is that when a broad range of less-niche (as presented, as advertised) options arrived from mainline brands, a wave of interest from the general driving public built, then cresting as batteries closed in on the end of their effective service lives, with all of the looming cost realities that go with that. Market interest then gradually redirected back towards the established cost dynamics.
Less than 5 years for most, when you get down to it.