If OP is lifetime Plat and has favored Marriotts for 17 years, but is now "disgusted" over a single minor incident of missed points due to a stopper well-known to most (e.g., the risk of booking outside marriott.com) and considering "switching my loyalty to a company that might value it," I have three observations:

1. Welcome to the 21st century -- the book-through-us rule is how most of these programs are structured.
2. Bonvoy is not a loyalty program, it's a revenue reward / alternate currency program, and when a chain pays a vig to a third-party OTA or corporate arranger, it takes a revenue hit. If you stumbled into the same Marriott on an opaque Hotwire Express deal, you'd be in the same boat.
3. Wow, pretty fickle -- if I were a Marriott strategist I would probably not go too far to retain this account.