There is no incentive if the technology is mandated
Firstly, there are six national multiplexes, not five. There are five SD multiplexes and one HD. At the moment. Ofcom are running a competitive process to launch two new ones.
The idea of the incentive pricing is to encourage the spectrum to be used efficiently. However, there is no point applying an additional tax if the broadcasters' hands are tied on becoming more 'efficient'. The spectrum plan and technology for Freeview was set in stone by government: the public service broadcasters had to achieve 98.5% population coverage, the BBC had to free up its second multiplex to convert it to HD mode, the majority of viewers had to be able to use existing aerials fitted for analogue reception, and we had to fit into the internationally-co-ordinated frequency plans. That really meant a requirement to use the 64QAM, FEC 2/3, 1/32 guard interval mode that the BBC and ITV/C4 are using. If they change that mode, to get more capacity and become more efficient, coverage will be reduced. The limits of what can be crammed into the 24 Mbps available have been pretty much reached, without reducing quality any further. There are already criticisms from many viewers that many channels are unacceptably low-quality, running 16:9 broadcasts at a resolution intended only for 4:3 pictures (544 x 576 pixels) and at a low enough bitrate to prevent the normal smoothing of macroblock edges to work properly.
The HD technology - DVB-T2 and AVC/H.264 encoding - can also be used for SD services, but are only viewable on Freeview HD receivers. The majority of viewers don't have one. The two new multiplexes - to run in this mode, and give four or five extra HD channels on each - are intended as an additional incentive for viewers to go and buy a new receiver. If a majority of viewers haven't done that, it won't be politically acceptable to turn off DVB-T/MPEG-2 support, and that will make the release of 700 MHz very difficult as there really isn't space for six national multiplexes in what remains. Viewers will be seriously angry if they lose services due to this - as it is, there are many people upset by the fact that they can't get three of those multiplexes if their local relay is PSB-only.
It won't be politically acceptable as people will expect the government to fund replacement equipment. For switchover, enough people had voluntarily switched that the government could get away with only subsidising equipment for pensioners over 75, the disabled, and other groups on long-term welfare. It was funded by increasing and top-slicing the TV licence fee, but only by a small amount as so few people were covered.
Meanwhile, the mobile phone networks are now running three generations of technology concurrently, with no end date for 2G announced or even considered. Phone still rely heavily on the 2G network for basic communications, as the promises of 3G coverage were broken and eventually the coverage requirements have been removed. O2's block of 800 MHz spectrum comes with coverage obligations - 90% of the population, if I recall - but the rest of the recent 4G auction has no obligations attached at all. It's still unclear if Voice-over-LTE even works, making voice services still dependent on 2G in much of the country.