Probably get removed. Even in remote places, I've got Three's 3G signal as strongly as I would have got O2's 2G.
If they deployed 3G on 800MHz like O2 have done in some places, it would work on all of Three's old 3G-only devices. Then you would also have 800MHz on 4G which means that Three would be the best indoor network coverage wise.
I'm not talking about consumer phones here though. There are many m2m 'things' use that 2G and will need it around for a long time. Embedded SIMs are used in many places; for example cars, emergency communications and smart meters.