I'm not aware of any hardware limitations with the current hardware unless it's for Apple Arcade. I'm not sure how it compares to the new 2019 Shield, but I still think the ATV4K is more powerful. My guess is we see a new Apple TV next fall at the earliest, or perhaps even not then. To encourage people to adopt a new ATV it would need to be able to do something the current one can't. Unless some things are needed for Apple Arcade, the current hardware isn't a limiting factor.
7.4.4 Theater Room: JVC-RS500, Silver Ticket AT 2.35:1 142”, Onkyo RZ830, Anthem PVA-7, Panasonic UB420, Apple TV 4K, JBL Studio 530’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, Roku Express (2019)