Many of the use cases I have that require a solution like EVS (e.g. a 5v5 esports event where we have 10x player feeds, or 10x player cams) are due to needing a 10 in / 2 out configuration.

I'm just curious what the reason is for this not being available in vMix Replay? A perceived lack of popularity/demand for that? Concerns about user hardware requirements? UI scaling? Something else?

That was one of my guesses, but as someone with hardware who could handle this, I'd really like to be able to do it. And was hoping devs/product might chime in with their thoughts.

Not sure if this works for you, but whenever I needed to record more outputs than possible, I ran vMix in 4k resolution and created a multiview on an output for 4x 1080p or 9x 720p matrix... You can also add one or more of the 4 outputs as a replay source, or you create a 2x2 matrix view externally (i.e. using a DMON Quad) and take that as another camera input.
You will then need to do some scripting, resizing and cropping for the replays to create a good workflow, but it should be possible.

I know it is not ideal, but if you really need the functionality that might be an option.

Another solution would be to add a second PC just for the additional replay cameras, send the cameras via NDI, and operate everything in sync using companion, macros etc.