I just updated a 1 day app, it took a couple hours. No Vue.set/this.$set, significant changes to render functions, changes to much of the boilerplate. I was thinking that most of the deprecated behavior could be easily emmulated. The performace improvements are worth the pain, but I have a lot of code to update. Hopefully the migration build will solve it. It would be nice to avoid a python3 situation.
On top of that, v2.7 slated for 2021Q1 also includes the relevant migration warnings, which should help make life easier