Switching one of the current commerical DVB-T multiplexes to DVB-T2 would create some additional capacity, but they're now full of channels from the big broadcasters, who are unlikley to want to take the hit that will come from losing viewers with older equipment.
Arqiva/the broadcasters have really missed a trick here. COM7/8 were launched to drive the uptake of HD/T2 receivers, with the intention of switching the other muxes to DVB-T2 ahead of their closure. Obviously that's now not happening, and the whole thing has been a bit of a disaster.
The trouble is excluding the PSB channels, COM7/8 never really picked up the major HD channels that would drive the uptake. Even in the case of 4's channels it's only carried the lower rating 4seven HD and C4+1 HD, rather than the HD versions of E4/More4/Film4 which would have had wider appeal.
The irony is with HD being the norm for a while now and the popularity of smart TV's I imagine most homes do now have a DVB-T2 compatible TV. Really this switch off should have been used as an opportunity to switch at least one of the SD muxes to T2, if not most of them.