It's not just a question of overall EROEI. Building wind farms requires most of the energy to be invested up front, whereas the return on that investment happen

It's not just a question of overall EROEI. Building wind farms requires most of the energy to be invested up front, whereas the return on that investment happens over decades. This means that the worse energy availability is right now and the more painful the other things we have to stop doing to make some available to build renewables, the more expensive energy from those renewables will be. One consequence of this is that demands from groups like Just Stop Oil to cut back fossil fuel production before replacements are ready will make constructing those replacements less viable, basically permanently locking in higher energy prices, worse availability, and more poverty. (For some proponents this is definitely seen as a feature; there's opposition to using technology to continue to maintain our current lifestyles.) As you say, energy is fungible, which means that if we do actually manage to build sufficient renewables first this in theory becomes much less of a problem because we can use their output to meet our energy demand and offset the decline of fossil fuel production. The economics of nuclear power are somewhat similar, whilst I think fossil fuels with poor EROEI are a little different in that more of the energy consumption is spread out over the production life. Also, the UK has unfortunately been struggling to build new nuclear like most countries, and the amount of time construction has been taking would be a huge issue even if the EROEI was good.