I agree that 5 GHz is preferable for wireless connections, but it isn't a requirement for the device to be 802.11 AC. Roku has had dual band wireless all the way back in a few of their first generation boxes (these new players will be 7th generation). Even with what you can buy today, the Streaming Stick, Stick + and Ultra are all dual band players, and those happen to support AC as well. But my 4th generation Stick (4th generation, model 3600) is dual band but only on N, and even older devices that are still in use have dual band N radios (Roku 2 & 3). Even some older ones do, but not too many of them left still in use.
Good point. You are correct that 802.11n can be dual band and that previous Roku devices have been 802.11n dual band. In the case of the new Premiere (3920) and Premiere+ (3921), the specifications seem to indicate it is single band 802.11n which means going back to only supporting 2.4Ghz networking.
And 802.11N is still capable of speeds in excess of Fast Ethernet (100 BaseT). Even the slowest routers are 150 Mbps, while the faster ones are up to 300 Mbps. More than enough for online streaming, and even good enough for local UHD content that isn't compressed.
Comparing the theoretical maximum wireless speeds with the rated speed of wired ethernet is not taking several factors into account. For example, the statement that even the slowest routers are 150 Mbps is technically true given they should be able to burst transfer up to 150 Mbps. However, if you look at reviews for wifi routers, you can find information such as the Linksys E1200 while capable of negotiating at N300 comes out in CNet's review as averaging 36Mbps when clients are only 15 feet away. Most Fast Ethernet switches I have worked with using category 5 wiring or higher usually average around 80-90 Mbps. Ultimately, the slowest 802.11N access point are *NOT* going to out perform a wired fast ethernet connect. Also, while even 36Mbps does exceed the 25Mbps recommended by Netflix for UHD streaming, CNet's tests don't take into account the impact of additional 2.4Ghz interference. You may still discover that your neighbor's baby monitor makes for a really bad time with streaming.
The other thing to take into account when comparing wifi speed with ethernet switched networking, wifi is a shared resource. If you have four devices of A, B, C and D connected to a switch, a transfer data transfer between A and B usually will not be impacted by a data transfer between C and D. The same can't be said about four wireless devices connected to the same access point. An additional transfer between C and D in a wireless environment does consume bandwidth from the shared pool of bandwidth across all the clients.
These factors are important when taking into account if a Roku Ultra is worth the extra cost. The answer is not as clear cut as that modern wireless is now rated faster than fast ethernet. The 100 Base-T ethernet port on the Roku Ultra still has value in 2018. There are situation were running ethernet cord through a home is not possible and for that situation wireless becomes the best alternative. But when wire can be run, nothing beats a wired connection in terms of reliability, low packet loss and low latency.
Also, for anyone that already has coax wiring run through their home and wants to avoid running addition wires, I highly recommend the Actiontec Bonded MoCA 2.0 adapters (model ECB6200). I really think Roku should consider making this product part of their own accessories store.