One very high cost to staying on 2 will be that the pool of 3rd party modules you can draw from will continue to shrink as projects drop support for it. Sure, you can stay on Requests v.whatever, but eventually the SSL certs that it includes will expire, and then you'll have to maintain your own patched Requests with the new certs you want to use. Or eventually someone will find security problems with Django 1.11 LTS, and will be on you to fix them. Also, every Django plugin will have migrated to Python 3, so you won't get to use the new tools for it that everyone else is using.
And at some point, say 10 years from now, you'll reach your breaking point where you'll get tired of maintaining backports for every. little. package. that you want to you use. And when that day comes, then you'll face the even more daunting task of porting your stuff to the new current version of Python.
I'm saying this as one techie to another: don't do this to yourself. Not long term, anyway. If you want to support your own ecosystem for six months or a year as an exercise, sure, I'll be the last one to tell you not to work on a personal project you enjoy. But for Pete's sake, don't engage in this as a long-term project. This isn't how you want to look back on your deathbed and see that you spent a decade.
As a practical matter, I'll say that everyone I've spoken to seems to have the experience that Python 3 seems like a pain in the neck for a very short while as you go through the process of finding all the bugs in your codebase that Python 2 was letting you ignore. But once you've done that, you're not going to want to go back and un-fix them even if you decided to stick with Python 2, and if you've already gone through that work, why not enjoy the new language features and 3rd party packages now available to you?