Why is Google calendar making it impossible to revert back to the legacy view? Why do they feel it's necessary to take away my choice and make the choice for me?
To answer your first question - you can't revert because they are not supporting the old version anymore. They've been saying for a while that they will be phasing out the old calendar UI.
I'm assuming they don't want to support two versions, which makes a lot of sense. There's no reason to support the old and the new, especially when the new version was probably programmed in a way to make future updates easier.
Google has also been working on rebuilding all of their applications to match their "flat" design. This can be seen with the new calendar app, but also Drive, inbox (compared to Gmail), Google sites, Android apps, the new fuschia OS (which is really just in the very early stages and nothing may come off it), and others.
As for your "why do they make the choice for me" question - this is just how the web works. Things get outdated quickly and apps change. When you're not paying for an app that's being provided to you for free, they do what they think is best. If you don't want the app to change, you've got the option of going for a paid alternative, or trying to find a free app that you can control the versioning of.