Logius is the company that actually owns and manages the DigiD stack, it's just that they hired Solvinity for their expertise. AFAIK Solvinity can't access the data.

I can't find it right now, but on Tweakers there was a long comment by someone on the inside that explained Logius basically had almost no know-how of how the current stack works, and there's lots of bespoke stuff. Basically classic vendor lock-in. The government (rather, Logius) now really wants to transition away from Solvinity, but that will likely be a 5+ year process.

I also feel like this is another thing that the "fast ring" of the EU should do together. Take Estonia's stack as a base, and then countries like Sweden, Denmark, Finland, The Netherlands adopt it and co- develop it. Make it extensible for the bespoke things the countries need, and every few years check which bespoke extensions can actually be generalized and modularized. Would lead to a much better product. A man can dream :)