National Roaming + sensible roaming fee = different story
National roaming doesn't necessarily make coverage 'somebody else's problem'.
If operators are mandated to offer national roaming (without charging extra for roaming calls), and if the roaming fee is set suitably high, then you create a sensible market.
Operator 1 doesn't quite have a case for rolling out coverage in notspotX, but if they're going to get roaming fees, then suddenly the business plan looks better.
Operator 2 was considering building a mast at tinyNotspotY, but they already have a mast from Operator 1, so it make sense to just pay the roaming fee
we have two (arguably) good properties here
1) greater incentive for at least one operator to serve blackspots
2) shared cost model for low-population areas where it really doesn't make sense to have coverage from multiple networks.
Property 1 does increase the overall infrastructure cost (which ultimately the customer has to pay) - but it delivers wider coverage
Property 2 should decrease overall infrastructure cost (by encouraging some sharing)
the critical thing is to set the roaming fee correctly. That would be a tricky one for Ofcom.