They did that for the second gen meters.

"The first covers newer, second generation (SMETS2) meters, where only the communications module needs swapping out, but even this is likely to cost £1.3 to £1.4 billion (c $1.7 to $1.9 billion) before the hardware cost of each new hub is even factored in.

The other is for an estimated 7 million older, first-generation (SMETS1) meter installations, which have the cellular comms hard-wired in, so the whole meter typically needs replacing. The report authors estimate this will cost another £2.1 billion to £2.45 billion ($2.8 billion to $3.3 billion), at an indicative replacement cost of £300 to £350 per installation."

That would've the smart choice.

They also could've designed all of this to work as a mesh network so they aren't reliant on anyone elses network.

Yes mesh networks have their flaws but they probably only need to check each meter once a month and it's probably just a MAC address and text. So speed and having the device constantly connected isn't necessary.