And when you have almost a million lines of code dating back 10 years; about 3% test coverage; and an executive team breathing down your neck to implement the features that should have been prioritized 5 years ago by the "leadership" they bought the company from?
When you have 3 engineers, one of whom is working full time on bugfixes and preventing everything from catching fire and one of whom is working full time on applying the second of 8 major versions' worth of upgrades to your framework (along with third-party dependencies that have to be version-compatible) just to catch up to a version that's still maintained with security patches?
When every framework/library upgrade, let alone language version upgrade exposes subtle backwards-incompatibilities with your external services, so you have to upgrade those too, and then spend 2 weeks understanding and solving the resulting forwards-incompatibilities?
Just because it was easy for you and your codebase doesn't mean it's going to be easy for everyone else.
I'm not saying we'll never doing it. I'm sure as hell not saying we (being the engineering team) don't want to do it.
I'm saying we haven't done it yet because we can't do it at all until we complete the other upgrades. We can't do it at all without spending a week or three months or however long finding or writing replacements for crappy (and in a few cases not so crappy) abandoned third-party packages that aren't and never will be supported under Python3. We can't do it until we can spare 3 months from our timeline to exhaustively click-through test the damn thing and verify we haven't broken anything.
And saying we had 15 years to migrate is bullshit. Our major dependencies haven't been stable on Python3 for more than about 3 years now.
By the way, did I mention before that we have other business priorities? Python2 is still supported. It'll continue to be supported for a few more years. Why should we spend valuable time converting to Python3 now, when the other things we're working on could be the difference between whether or not we're still around in a few more years?
No, it won't be more costly in 2020. It'll be less costly. We'll have a larger engineering team because we solved more business problems and customer problems, so the business earned more money and could afford to hire more people. Or we'll already be out of business and it will have had nothing to do with which version of Python we were using.