The argument of go 100% FOSS or not reminds me of the argument of free market vs regulation. Yes a free market sounds ideal, but in practice, a few major players take advantage and everyone who isn't them is left out in the cold.
The same applies to FOSS vs Proprietary software. If everyone did FOSS to the fullest extent, and anyone building products on FOSS made their product 100% FOSS, then it wouldn't be nearly as big a deal if Amazon took TimescaleDB and sold it, because anything they did would be available to everyone else.
But that isn't how it works. They just wait until a market establishes itself and swoop in and do something someone else is already doing successfully, but better by using their massive resources behind it. Honestly it's a lot like embrace, extend, extinguish. I'm not saying this to shit on Amazon, I don't necessarily think it's intentionally malicious, it just often ends up hurting a lot of organizations building FOSS.
I think of it this way: A lot of people agree that tech giants are increasingly becoming more powerful as they expand their reach, often due to their ability to simply buy or kill the competition through undercutting with their massive cash stockpiles accumulated through their primary business.
For FOSS, it is the same strategy except the competition isn't being "bought", it's simply being taken (FOSS) and made into something "better" with their nearly unlimited resources, and then undercutting the original proprietors.