There's something about the current situation that makes me feel the existence of NATO holds things back. I'm not arguing here to get rid of NATO (??) but...
- if there were no NATO I believe some countries may have gone in with troops, or at least gifted jets, to aid Ukraine. Maybe Poland and the UK? Yes, Russia may have declared war on them, but then others would come in also: US plus... A gradual increase in the size of the battle against Russia.
- with NATO it's either all in or nothing
That's an interesting thought. If, as in your example, Poland and the UK decided to get involved, and Russia then declared war on us, and therefore other countries (US plus) got involved, then basically we've ended up with the 'all' option. So it's still ended up as an all or nothing scenario, just that there was a short period of time where it was 'some'. The danger with that is that it didn't seem like an all or nothing scenario, which makes it more likely that you'd turn down 'nothing' and end up with 'all'.
And that's ignoring the fact that Russia could have nuked Poland if they got involved, and warned the rest of us to stay out or we'd get the same.
Maybe the fact that we know it's all or nothing, as does Putin, means we're going to work longer and harder on a diplomatic alternative, and sanctions etc. There's no saying yet that they won't work.