I wish the US would be held accountable to the same rules as every other country, but I don't think what you describe would happen. I don't think anybody wants to invade the US when the US has the most powerful military on the planet.
As for NATO losing capabilities if the US leaves, sure. But it looks like the US doesn't really care about helping defend Europe anyway. Maybe at some point Europe will just have to focus on its own defence without the US. You say it would be "morally wrong" if a NATO country (e.g. the US) wouldn't fulfil its NATO commitments. Do we think Trump cares about morals though?
If the UK is serious about creating "$100 billion-plus" companies which rival "Microsoft, Google, Nvidia and the like" then maybe the UK should rejoin the EU and help to create big European tech companies.
Look at Airbus, a big European company which does very well. They're registered as a European company rather than a company of any particular nation. They have operations across Europe. Maybe that large scale is what allows them to be so successful.