In the 90s I used the ibm? (I think) boot manager with 4 or 5 OSs, it took a one or two megabytes and worked quite well. That is more choices than are practical, honestly. And with VMs and containers less necessary than ever. (I do like GPT.) Doesn't have to be that small today, but shows what is possible. Xosl too.
Thirdly developing a completely new proprietary OS (uefi) when embedded Linux exists is about as cynical and wasteful as they come.