Being picky here on a few points:
The “five years” is a tentative schedule. The FCC has reserved the right to shorten or extend the simulcasting requirement at any time, at their discretion.
Low-power broadcasters don’t need to simulcast. All current NextGen TV stations are low-power, because of not needing to simulcast.
Only the “primary” (essentially the nn.1) stream needs to be simulcast. Subchannels don’t need to be.
The “ATSC 1.0 simulcast” requirement doesn’t include any requirement to be HD. Due to limited bandwidth, it’s almost certain that most of those will be reduced to standard-definition 480i.
The simulcasting requirement is a real problem for broadcasters, because they aren’t getting any extra channels like they did for the digital TV switchover. You can’t run both NextGen TV and ATSC 1.0 in the same channel – they use different modulation standards (one company is touting they’ve done it in the lab, but they used a third modulation technique). Stations are going to have to get together to share bandwidth. Sharing the NextGen channel shouldn’t be too painful, due to the extra bandwidth available, but fitting all of the data from multiple stations into one ATSC 1.0 channel is going to be problematic if HD is retained.
From Wikipedia: “[The FCC has] suggested that multiple broadcasters in each market cooperate by locating multiple degraded ATSC 1.0 services on a single transmitter.” (emphasis added)
From the FCC (link is Word document, sorry): " We decline to adopt requirements regarding the format of the 1.0 simulcast signal. We recognize that broadcasters may face spectrum constraints that could limit their ability to continue to provide HD programming or other enhanced formats on their 1.0 simulcast signals."