Some possible downsides (for larger established .co.uk brands)...
- They already have .co.uk in their logo. For a small business this might be a small cost to amend and recirculate, for a large brand even a small tweak to a logo becomes an expensive update across collateral.
- They've already printed .co.uk on every instance of the domain name in use 'in the wild'. From printed materials to vehicle signage to billboards.
- They may find their CTR drops in the SERPs, because the .co.uk extension has been promoted by the brand since the inception of their website, and the general public are told to be cautious of domain extensions they don't (yet) recognise.
As you say, they could just 301 from the .co.uk to the .uk, but there's a possible downside to that too...
- 301'ing a huge website from .co.uk to .uk is another expense, sure a small site is simple enough. But if you've got a monster of a site with various URL structures etc then you're potentially adding to the cost and maybe even the load time. A brand like Amazon... there's going to need to be upsides to outweigh any time implication, no matter how small. There's also the server overheads for delaying the site load with a 301 for huge traffic sites.
- Very few of the biggest UK sites 301 their .uk's.
Clearly there are downsides, otherwise we would have seen far bigger sites switching to the .uk. After all they are out for every competitive advantage available to them. Switching to .uk sadly isn't a competitive advantage for sites already established on the .co.uk (in most cases, so far).
Right now it's still; Risk > Reward.
Just about every argument in this thread was raised in the debates for launching .uk, when it was first proposed. Sadly it fell on the tone deaf ears of the Board, who were more focused on printing money for bonuses and the like at that time. I think they could have found ways to give it a stronger start, but I'm sure there was less revenue in those ideas.
I think the shorter extension suits some startups and tech focused companies (like the web hosting examples referenced earlier in the thread).