After all, if a war broke out and our normal trading partners weren't willing to sell us ships, presumably they wouldn't sell us steel or engines or ball bearings or paint or radar modules or computer chips or plastics.
The real answer to this is threat analysis: what are the realistic scenarios under which it becomes a problem. e.g. if Japan stops supplying ships a) how likely is that and b) could we just buy them from the Dutch instead? However, the stakes for getting this wrong are high.
But the UK decided against that in 2016, and despite demographics having shifted the views, there is no general "rejoin" campaign.
I think rejoin is going to be politically unpopular for a while, as there's no way we could rejoin the EU on anything like as good terms as we left on.
The UK is too large to be an independent city state, but too small to be self sufficient.
The problem is the UK is an oligarchy owned and run by non-dom billionaires. It's essentially a floating labour camp. Virtually all the major industries are foreign-owned, and profits flow out of the country instead of being invested in infrastructure, R&D, and national development.
The public school/Oxbridge educational path is based on a 19th century view of the world, which has its roots in a feudal society that expanded into global piracy.
Rejoining the EU isn't just about undoing the direct, about exposing the population at all levels to a more modern culture with elements of social responsibility and rational state planning.
The EU isn't perfect. But Europe successfully rebuilt after WWII, while politically the UK is still stuck in the 1930s.
And while the majority agree we should never have left the EU, and the majority agree we should rejoin, that isn't translating to political support.
Indeed the plurarity of votes is to Mr Brexit himself, who thanks to FPTP is likely to get a majority government in 2029 off 30% of the vote.
Hell 1 in 8 people that voted Farage last year think the UK never should have left Europe. Not that it wasn't the right type of brexit, but that it shouldn't. Yet they voted for Farage, decided that was a mistake, and are continuing to vote for Farage.
I get it. Some people just want the world to burn.
But a similar number think we should rejoin the EU, yet voted Farage last year.
It's no wonder support for democracy is at an all time low.
On day one - for past bad behaviour.
If your usual trading partner is inaccessible for reason X , what are the odds your alternate trading partner is also affected by reason X? Are there geopolitical or national political reasons that partner B might become unavailable or unpalatable at the same time?
Otherwise sure, realistic threat scenarios. But the world is also changing fast. Denmark or Canada did not expected to be militarily threatened by the US some years ago and still this is where we are now.