Re: The BBC had their chance ...
It wasn't the digital switchover, it was the utter, utter failure of a subscription model for terrestrial digital TV. ONdigital and then ITV digital both failed to make it work, despite all boxes having card slots and an ongoing battle with the "pirates". At the time I seem to remember most of the discussion was around how poor the signal was (dropouts) and the fact that for your couple of hundred quid outlay (in the early days) and monthly subscription, the number and quality of channels available wasn't compelling.
Digital Switchover was set for 2012 immediately after the Olympics because the government had already sold off the "800MHz" band and mobile operators were desperate to get services up-and-running, which they couldn't do while analogue TV still co-existed with digital; that band was required for transmitters.
Given that two commercial ventures had failed to get the thing working, that uptake had been slow in the early days because of problems with low power transmitters, electrical interference, insufficiently well-designed RF "front ends" and a sheer lack of a "killer app", unless the government wanted to nationalise the network, the only thing they could do was to hand the whole thing over to a loose collection of others with the BBC in the lead - technically if nothing else - and give the licence fee a small boost so that they had the funds to sort things out properly.
Moving from 2k carriers to 8k carriers improved robustness (a longer "symbol period") but rendered some of the very earliest boxes obsolete, which was a blessing in disguise because newer boxes had much better RF circuitry. Upping the power of transmitters where possible also helped.
Once the signal was robust, moving from 16QAM to 64QAM increased the payload available (more channels) and mandating that nearly all the available space on the four "important" and two "less important" multiplexes was given over to free-to-air channels meant that suddenly the free offering was seen as reasonably good value alongside a basic Sky or cable subscription.
By this point, a basic decoder could be picked up at Tesco for £50 or less, and PVRs were becoming affordable choices for the majority, rather than expensive toys for early adopters.
All of which is a long-winded way of saying that you are probably correct about the card slot. It was never in the BBC's interests to mandate a card slot and one of the biggest practical arguments against moving to a subscription model now is the sheer installed base of incapable TVs.
In fact the most practical way of moving to a subscription-only model would be to switch off terrestrial transmitters altogether and mandate that everyone either receives TV via satellite (all, or at least most, satellite receivers have a card slot) or over an IP connection. I have no doubt that the government would love to do that. They sold off 800MHz in 2012, 700MHz is being cleared as I speak, so making the 600MHz and 500MHz bands available too could be "nice little earners"...
Pity those of us trying to use UHF radio microphones and the like.