The same thing happened in the US. With the arrival of lagers and especially pale lagers, other styles did their best impressions of the king. We had cream and steam beer, sparkling ales, extra pale lagers, and so on. Bocks, which ranged in color from the pilsner-pale to porter-brown, were always strong and rich. In Germany they are by law starkbiers--or strong beers. As America was turning to very light-bodied, low-to-medium alcohol pilsners, bocks were finding themselves out of step with the trends of the day. So they started to slim down in an effort to appeal to the modern market.
When I was writing about bocks for the book, I had to contend with that tiny remnant of a style that had evolved to fit older tastes. Call them old American bocks, I guess--Shiner, Huber, Genessee, and Yuengling. They range in strength from 4.4% to 5.5%, and three of these four are 5.2% or lower--in many cases weaker than standard American pale lagers (Budweiser is 5%). Obviously, nothing stark about that. But they are a kind of uniform dark amber, and I think that's what once communicated strength. It's hard for younger drinkers to imagine, but back in the 70s and 80s, people reflexively assumed dark beers were strong. Guinness, people would regularly tell you, was a titan.
(It led to bizarre myths, like the one that said that bocks were produced by scraping the dregs off the bottom of the barrel. Modern brains, familiar with the brewing process, don't even know what that means. I certainly don't. But I think it goes to show that anything that wasn't pellucid and straw-colored was suspect. Look at grandpa's crazy dark beer--eww!)
But now these beers are trapped on a vanishing island of misfit styles--too weak and insipid to attract people who want a real bock, too old-timey and "strong" for people who want a lite beer. Their market must consist of a dwindling number of old drinkers who remember the beers fondly. I suppose they could be rehabilitated via retro nostalgia (the funny old goats on labels are poised to beguile), but as an extant style, my guess is it's not long for this world. Go get your