You can not know if Migadu will increase their prices again, just like you can not know if Scaleway will raise again (they raised 3 times in less than two years); but you can also not known that MXroute will not rise theirs prices too.
And, mxroute has a lifetime plan.
Nevertheless, glad you found MXRoute fits your needs and budget!
I think, unless you acknowledge the issue, it's hard to have a lot of confidence it won't happen again. And that's not to mention all the licensing language people mentioned elsewhere. It just seems like using your service would require me to read all the legalese and revisit your website at some frequency to make sure nothing's changed.