There were many factors. Rails itself isn't too bad to upgrade, but the problem boiled down to various Rails engines (active admin, etc). There came a point where I sat down and had to decide if it was worth de-tangling the mess and potentially breaking a lot of things to upgrade Rails and the various dependencies (only to do this again in 5 years), or possibly build things in a way that would work regardless of the dependencies.

Sure-- the rewrite was more effort up front but the hope was the long term compounding benefits would be worth it. I don't generally advocate for rewrites but in this case I went for it (and actually got it done).