Imo the right way to think about nostr clients might be that they are a kind of web browser. Are there any web browsers you would pay a subscription for? Probably not. I think the question should be, what businesses can be built on nostr (nostr which is, afterall, the worlds largest bitcoin economy), not what nostr clients can be a business. If businesses are making money on nostr, they will be happy to support high quality clients that bring users to the network. Just like companies and foundations support web browsers. Or, maybe a company in the bitcoin space would like to create a free nostr client as way to strengthen their brand, the way Microsoft or Google do with Edge/Chrome That being said I think in the long run anything anyone can do to get more people on nostr and more zaps flowing will be rewarded by the bitcoin gods.