Three already has a decent network, and a good backhaul. I do believe it can build a pretty solid network, especially with 800. It may not offer the highest speeds and may have to manage traffic, but it will still be a solid, dependable, network.
If it ever happens that 3G spectrum is refarmed for 4G then all the better. And in a year or two, I expect the VoLTE issues will be solved and almost everyone will be able to use it, including those with SIM free phones.
The network is clearly under pressure now, and I hope that the extra spectrum will make a very quick improvement given the fact that 3G data can be much faster, so I'm suspecting congestion at the site, not the backhaul.
For the consumer, it's great that - at last - every network is eventually going to offer solid 4G coverage, but there will then be a lot of continued investment to be done or it will just be history repeating itself.
4G will evolve, and people will want faster speeds, so will O2 or Three be able to keep up with EE and Vodafone? For M2M, LTE will advance to offer lower ping times and reduced power consumption - and which networks will be ready for that?
It doesn't just end when coverage gets to 98%, and there's also the issue of cost. Does everyone want to pay the premium to EE for those faster speeds? Some will, some won't.