This is a common misunderstanding. There are plenty of alternative locations to mine rare earth minerals, particularly Australia. China cornered the market because it's a high pollution low margin business. If geopolitical concerns cut off access to Chinese sources, alternatives will be developed.
I don't think this is the right way to characterize it. China invested when other countries didn't, but they didn't monopolize the market, they have no moat beyond expertise and some tech advancement that could be replicated easily enough. The only moat they have is related perseverance and other countries simply not wanting to put the work in.
Basically, if we want to replicate what they did, we will have to do it mostly from scratch -- Japan and Australia has done some of the work already so it's not totally from scratch. It's obviously not impossible but it could take almost a decade for us to do that.
That said, I don't think this should be enough for Japan to stop investing in EVs. If Japanese car makers are really worried about this then they can build their plants in the US and leverage any deal the US has with China on real earths. They've already starting importing Japanese cars made in India and the US back to Japan so that's an established practice. Then once they've secured their own supplies they can make the EVs in Japan too. I think OP's point about the suppliers have more merit as a reason why Japan might not want to develop EVs.
Moat is decades of process / tactic knowledge built by disproportionate amount of talent on geologic formations others didn't invest in. Right now they generate 15x mining graduates, university of mining tech alone enrolls more than all US mining programs combined. Then you throw all that into a mining city like Batou with 3 million people running vertically integrated operation. That's ecosystem scale with compounded advantages beyond "wanting" to put work in, it maybe scale on PRC has demonstrated ability to produce.
Between shallow kiddy pool and Mariana Trencth in terms of ease of replication, I wouldn't lean towards kiddy pool. I don't think "right way to characterize" their lead is "no moat" beyond... all the things that are actually, in fact very deep moats, as if any country can persevere their way to replicate decades of work and execute industrial policy of a 3 million large city dedicated to mining/rees.
I surmise, PRC will build out EUV (technical problem) and produce them at scale before west+co meaningfully tackles HREEs supply chain (technical and regulatory and industrial problem).
Incorrect, de facto, the only firms invested heavily in the rare earth refineries technology are Chinese for the last 20-30 years. Their moats are as deep as TSMC moats so to say.
See my sibling comment. Their moat is the scale and structure of their industry. Some parts of rare earth processing are dependent on that.
Well, well, well, if it isn't the consequences of everyone's own inaction...
I do think the original point about lower complexity vehicles being a threat to the suppliers has some merits though. Germany faces a very similar dilemma and made similar decisions.
Incredible what can be done. If Japan ever wants actual mecha warriors for their military, they're going to need motors like that.
