"cancel your contract with EE, (good luck doing that, by the way), and buy a SIM from Three, or anybody else."
After EE went 4G round here, me and a few neighbours who'd had years of decent service on Orange found the 3G service basically useless.
It did indeed take a number of months to leave Orange, and I moved to another network with decent coverage at home.
But then I started to spend more time in a place where the new network's coverage was as bad as EE's was at home.
Not only that, BT went and bought the new network company.
I think I'd rather have a monopoly (with decent coverage and decent service for a decent price) rather than a race to the gutter in price+service terms and a lottery in coverage terms.