You make a good point that some folks using "classic" are really using one of the page builder plugins that are incompatible with Gutenberg.

My long-term view there is that "proprietary," as in specific to a single plugin, data structures and UI are going to be less and less appealing to users over time as the standard rails of blocks and the applications being written on top of Gutenberg blocks, including the mobile apps, become more of a must-have for many sites.

The future of page builder plugins is extending Gutenberg, not trying to recreate what it does well with fewer developers and a much smaller community.

This switchover has already happened for a good chunk of the user base, over half, and the remainder will have an opportunity to switch on the cadence that they redesign their site, which is usually every few years.