Not for the first time, David Halliday has hit the nail on the head with this tweet -
"If not having an independence referendum before 2021 is sensible then why has Ruth Davidson been fighting so hard to make sure that that's what happens?"
If you find yourself doing (or considering doing) exactly what your opponents want you to do, it's always worth stepping into their shoes and considering why they want you to do it so badly. From a Tory perspective, there are a number of very good reasons why an early independence referendum is something to dread -
1) The imminence of Brexit means that Project Fear would work in both directions this time. It will be easy enough for the Yes campaign to produce a steady stream of "There were warnings tonight about the impact of Brexit on..." stories. (How easy it will be to get the broadcasters to give those stories equal prominence is another matter, but an official campaign can help set the news agenda to some extent.) If voters are convinced that there are credible reasons to fear the uncertainty of Brexit, the effect of fear in the campaign may be neutralised in a way that was never possible in 2014.
2) Theresa May is absolutely the worst person to be a figurehead for the No campaign. She is tone-deaf in respect of Scotland. She could single-handedly lose the referendum for No.
3) Jeremy Corbyn clearly has some appeal in Scotland, but an independence referendum would not be his natural terrain. As was the case during the EU referendum, he probably wouldn't look terribly interested. He would also say random things about "SNP austerity" that just wouldn't have much resonance for people in that particular context.
4) Given that the Tories are now Scotland's second party at almost every level of representation, it would be hard to justify sitting back and allowing Labour to be the cuddly public face of the No campaign once again. And yet the alternative - an identifiably Tory-led No campaign - carries enormous risks. Notwithstanding Ruth Davidson's much-vaunted "popularity", the Tories remain the most disliked of the major political parties in Scotland. In a binary-choice referendum, there's not much use having 25% of the population solidly behind you if another 65% hate your guts.
5) The Vow may be a trick that was only ever going to work once. On the pro-independence side, we tend to think of what could go right or wrong in a referendum purely in terms of victory or defeat, but for the Tories, giving too much ground on devolution is a fate almost as bad as defeat. If a Yes vote looked like a realistic possibility with a few days to go, they would have to decide whether to make very painful concessions of new powers, or whether (and this is more probable) to offer absolutely nothing and just hope for the best. Neither option looks too appetising for them in advance.