A couple of things worth considering are the revenue that it would create for rugby and the tax receipts it could generate for the govt.
On the first, I would expect total gate receipts for a home tour would be higher than they would have been, even in a normal year in SA. This raises the prospect of both the home unions and the SA union being able to generate some decent and much needed income. Cancelling the tour and having no income makes no sense.
On the tax front, every £1m of ticket sales would include £167k of VAT. Then there's all the booze, food, corporate hospitality that would generate further VAT sales, as well as alcohol taxes. All of this would be served and catered for by a sector that's been on the bones of its arse for the last 12 months and kept on life support up by tons of taxpayers' cash. Given the potential upside for the Treasury if the tour happens, asking it to underwrite it isn't exactly barmy. Let's be realistic, no insurer is likely to touch this with a socially-distanced bargepole and the unions don't have the spare cash to self insure.
I also wouldn't be surprised if the govt is promising to underwrite the hosting of the Euros. UEFA is going to need a single host, rather than having super-spreader teams jaunting all over Europe. And it's going to need a host that has a reasonable chance of having delivered widespread vaccination. Step forward the UK. But, as it is not without risk, so it'll need underwriting -- most probably by the govt.
In short, if you want to make decent money, you have to accept/take risk. If you want risk free returns, then expect lousy returns (if any at all).
The decision here will be taken in Westminster, so it should be fairly easy to get the devolved legislatures on side. The Union is paying and offering you bread and circuses -- so what's not to like? The Welsh will be onside as they love rugby, but Wee Jimmy Kranky will probably be a pain. Don't be surprised if she asks for a second bung through the Barnett formula and then claims that it proves how much wealthier they will be after independence.