The "weird licence which is basically a modified version of the MIT licence but with a clause that prevents competitive usage" is likely not even recognized as a true "open source" license.
> would it be possible to relicense a fork of Polaris to MIT (removing the Shopify clause?)
Likely not, because only the IP owner can determine who can do what with their IP under what license. If you use the software of an IP owner under a specific licence, you usually don't have the rights to re-license their work, even if you modified it.
> Then do you (developers on HN) stay away from CLAs?
Depends on the CLA, but generally I do stay away. E.g. I never checked in anything to the official Qt repository because I don't agree the the CLA by QTC. Instead I finally made my own fork and call it LeanQt and LeanCreator (see https://github.com/rochus-keller/leanqt/ and https://github.com/rochus-keller/leancreator/).
The "weird licence which is basically a modified version of the MIT licence but with a clause that prevents competitive usage" is likely not even recognized as a true "open source" license.
> would it be possible to relicense a fork of Polaris to MIT (removing the Shopify clause?)
Likely not, because only the IP owner can determine who can do what with their IP under what license. If you use the software of an IP owner under a specific licence, you usually don't have the rights to re-license their work, even if you modified it.