Here's a pretty informative story on how The Crew was developed and brought to PS4:
Ubisoft Reflections tells the story of how it brought the new Ivory Tower driving game to PS4.
www.eurogamer.net
Practically all games are developed on PC to some degree even if it's not a release target, because all the tools are on PC. That said, we don't know how much prior PS4/PS5 games had work done on PC by the first party team. The Spiderman 2 leak is the best thing we have to go by. I don't think we've seen any hint of Naughty Dog having a PC build of Uncharted 4 (for example) prior to the actual port announcement.
You're not wrong that simultaneous development affects development, requires some extra resources, etc, but it is very small in the scope of a $100M+ AAA production. If the budget on a Nixxes porting job is $2-5M, we're talking like 1-2M extra to maintain a PC port from the inception of the project AND the resulting quality will be better. It's a hell of a lot harder to come in as a contractor/support team and get to grips with a code base you've never seen before.
In the ideal situation, whether day and date release or not, first party teams will maintain their own ports from the start. The value of Nixxes is more like how Bungie consults on live service projects across Playstation. They can provide all kinds of PC-related best practices the console-oriented teams haven't considered before. Issues like how to handle multi-monitor, or simultaneous keyboard/gamepad inputs, or graphics settings, etc. Nixxes shouldn't be the ones doing a full blown port after all the code/assets are finalized. They're good at it yeah, but it'll never get the best result.
So then the issue is: you've got a high quality, release-able product on day one. Do you sit on it for 5-6 years while it drops in value just to appease people on message boards? Of course you don't.