I'm not sure if 4K UHD Blu-Ray's will ever become mainstream but I think they will continue to have a niche market. Hopefully cost's continue to be low.
Whether it's right or wrong, streaming will slowly take over, even for 4K, but the problem is they often do stripped down over-compressed versions that probably won't even look as good as a good Blu-Ray.
7.1.4 Theater Room (In Progress): JVC-RS46U, Silver Ticket AT 2.35:1 142”, Onkyo RZ830, Anthem PVA-7, Panasonic DBT-110, JBL Studio 270’s, 4 - 15" DIY Sealed Subs
3.1 Living Room: Samsung 64” F8500 Plasma, Anthem MRX 300, Dynaudio Audience 52’s, Dynaudio Audience 122C, NHT SubOne