Is it worth the headache of upgrading to 6.0? I'm looking at the change log and deprecations and nothing stands out as an issue but I think I still need to do 5.1.5 to 5.2 then to 6.0. Not to mention I'm not going to utilize any of the new upgrades.
If this works I'm hoping at least pushing to Heroku will handle that part. Heroku notes the gem file and makes the required updates.
EXCEPT that by not upgrading you are building technical debt, because eventually your dependencies will be unsupported, and have a severe security vulnerability, and nobody will be providing a patch. And they'll be hard to get running on modern OS's, etc. Or some _other_ dependency will have a severe security vulnerability, and will release a fix ... in a version that only works in Rails 6.0+.
In fact, Rails 5.1 is _already_ out of support in the Rails maintenance policy, they do not commit to even releasing patches for severe security vulnerabilities in Rails 5.1. (They forgot to update the exact versions given as examples in the Maintenance Policy Guide, so you won't see it in the "released" Guide for Rails 6.0, but you can see it in the "edge" guide https://edgeguides.rubyonrails.org/maintenance_policy.html. And next time there's a Rails release, it'll be updated on guides.rubyonrails.org.)
But will it make your software faster or more reliable to upgrade to Rails 6.0? Probably not. But this is how software works now, you build up technical debt just by standing still.
I believe there are security vulnerabilities in that version. consider updating to a secure version: 5.1.7 is the latest revision of the 5.1.x series.
https://weblog.rubyonrails.org/2019/3/13/Rails-4-2-5-1-5-1-6...
So you could consider upgrading to 5.2 instead of 6.0. This is just one minor version step so should be less daunting. And 5.2 will receive security updates for a while yet.
Always. At the very least simply because the headaches that accompany those upgrade paths will be significantly bigger once you skip a version or two and finally want to jump to 7.0 or 8.0. Instead upgrade while it's relatively painless.