Actually countering Russian military capacity really isn't
an issue any more, and Britain and France have enough
nukes to act as a deterrent
Which... okay, yes. It's distinctly better than not being safe from a preemptive Russian nuclear strike. However, this is merely necessary; not sufficient.
Have you considered the vast number of non-nuclear aggressions that Russia might undertake, and in fact has already undertaken?
Crimea was safe from Russian nuclear attack too. Ask them how it went.
Are France and the UK's nuclear arsenal going to help if Russia makes more Crimea-style land grabs? Shuts down shipping lanes? Cuts pipelines or internet access? Shoots down geostationary satellites over Europe? Assassinates the leader of a NATO nation?
Granted, there's no guarantee that the United States would help you then, either. We have not been the most reliable partner, and I am deeply sorry for that. I do not support the leaders who have made things this way. I want us to be a better partner.
Regardless, surely you understand that there is a great need for conventional military deterrence by NATO? Nukes solve an extremely limited list of potential problems.
But would they be willing to sacrifice their countries in a nuclear exchange on behalf of Germany? Also, Britain and France have far fewer nukes than Russia and the US. In addition, they don't have the massive land area to make a first strike implausible.
Russia is a threat, but in the annoying neighbours who keeps holding loud parties, deal drugs and litter the streets and who's dog keeps crapping on your lawn kind of way. You don't want to do anything to encourage them, but actually your uncle is in the police and you have two cousins in the Army, and if the neighbours tried anything really serious, reality would assert itself pretty fast, and not in their favour.
The US is keeping it's military presence in Europe to serve US strategic interests, yes versus Russia but also with respect to the Middle East. US operations in Iraq, Syria and Afghanistan are heavily supported from bases in Europe. Pulling out those bases would cripple US ability to support those operations and project power across large swathes of Euraisa and Africa.