Wait, what?? The items inside the ball valve are able to [easily] withstand boiling temps, and then some. The
3 piece stainless valves have Teflon (PTFE) parts inside.
Also, you take the thing apart AFTER you're done brewing, WHEN it needs to be cleaned completely. I normally just give it a good soak in PBW solution. Squeaky clean that way, with almost zero effort (my keggle ball valve uses a TC connection to the keggle). Otherwise, cleaning is easily done via a beer faucet brush (cheap and easily obtained).
IMO, trying to cut corners can very often result in more issues than not doing so. IMO, trying to brew/boil and then ferment in the exact same vessel is one such case. It's fine IF it's working for you, but there's no way I'll do it. IMO/IME, trying to store 3-4 (or more) 8-15 gallon kettle fermenters is not a wise thing.
Do what you like, but I don't see using the BK as a fermenting vessel, too, as a good idea. Or even a practical thing to do. I'm normally getting 6.75-7 gallons into fermenting vessel. In a 15.5 gallon keggle, that would be far too much headspace. Besides, I'd rather have the kegs as dedicated items than try to MM one into trying to do both brew and ferment.
You might think you're avoiding "thousand paper cuts"... Instead, you're playing Russian roulette with your batches. So trading paper for a live chamber...