although I would think that it could be released with the caveat that old saves won't work untill the full release)
Why wouldn't old saves work? There are only 2 situations when an older save won't work.
1. A new chapter is released, you play the alpha version and save at the end. Time goes on, another chapter is released and you try to use your alpha save, without thinking that the final version might have changed something.
2. Let's say that you have a save for chapter 9, but Eva announces that she had to change something that happened in a previous chapter(ch7). In this case, the variables from your save file won't be compatible with the new ones, hence, old saves won't work.
So yeah, unless you try to use a pre-remastered save, your files should work.
Nothing changes for those who want to wait for the full release as long as they are disciplined enough to not play the "half-release", it might even result in a better experience the the Ian part could already be revised/improved through feedback.
Something does change. It's not a matter of discipline for those that don't want to play the first part. I am sure quite a few people would be capable of that. It's more a matter of discipline for everyone else, to not discuss about the new content. In the end, you need to choose: live under a rock until Lena part is release and avoid everything (patreon, discord, this thread) or play the new release.
Anyway, it doesn't matter what are the pros and cons of releasing Ians part first. The people have voted and unless Eva will say in the next progress update (those posts should be renamed from weekly into progress) that she's going to release it anyway, we won't see any new content in the near future.