I believe that Next-Gen succeeds if it can demonstrate 1) that it fixes what's broken with the current ATSC standard, and 2) that it provides additional services that viewers want.
I think the #1 pain point with the current ATSC standard is its susceptibility to interference, which kills a digital TV signal. Viewers are shut out of OTA because of terrain or tall buildings either blocking the signal or reflecting it, causing multipath issues. Some broadcasters tried to address signal blockage using a distributed television system, or DTS, but that has failed miserably due to signal overlap again causing multipath issues. VHF is a challenge for DTV due to noise interference from home electronics, appliances, CFL and LED light bulbs, etc. Low-VHF is near-useless. Viewers have largely abandoned large outdoor antennas for compact indoor antennas for a variety of reasons, mostly aesthetic, but that brings home construction into the mix, with radiant barriers and chicken wire that forms the basis for stucco construction adding another source of interference. The FCC didn't help either by reducing the size of the TV band, meaning co-channel stations are located closer together than before, especially in the eastern half of the country. Mobile viewing is all but dead - the current ATSC signal isn't robust enough for use inside a moving vehicle.
If Next-Gen solves most of these issues, I give it a chance. A more robust signal with better error correction is one of its promises, and that will go a long way toward its success. I still think that low-VHF is a bust, due to the public's disaffection with large outdoor antennas, and I'm not sure mobile TV can be resurrected, but ending the issues with multipath will be key.
As for additional services, over-the-top, or OTT, is the big promise. Next-Gen is an internet-based standard and if broadcasters can seemlessly marry the base broadcast signal with an enhanced OTT experience, it might provide the sizzle needed to generate buzz in the general public. However, that will also require the broadcasters to avoid the temptation to data mine the snot out of the internet connectivity. Any hint of "big brother" will drive the public away from OTT. I know that proponents are trying to sell the advantages of 4K, but I don't see it happening. I'm told that a screen size of 65" or more is needed to really see the difference. Some are predicting no more than 2K. Additional subchannels might lure others in. Whether 4K or additional subchannels, those won't really come into play until each station is broadcasting exclusively in Next-Gen.
Phoenix should be a good test market in 2018. OTA viewership exceeds 20% here, due in part to large OTA viewership among the market's Hispanic population and the fact that both of the full-power Spanish-language stations are participating. Homeowner associations (HOAs) are present in the overwhelming majority of neighborhoods here, which discourages outdoor antennas, and stucco construction is predominant, providing a challenge to indoor antennas. Only two of the ten stations will need to change frequencies in the repack, and those will be completed by November 30, as both are in Phase 1. The broadcasters here have a good relationship with the cable companies (primarily Cox) so that will help.
Can Next-Gen succeed? I hope so, but there's no guarantee. Viewer experience must improve and broadcasters must be able to monetize the improvements. Otherwise, it's 3-D TV all over again.