Before the rise of services like JLCPCB, I would have agreed with you.
Services
like JLCPCB - are there really others? Mention one. Do they work? Are they future-proof? Do they guarantee the availability of parts?
To me, it seems JLCPCB hasn't yet changed how the industry works. We'll see maybe in 5 years. In the meantime, I apply all the old methods; I still have to build the prototypes, for which I still have to order components as the BOM says. I still need to manage the BOM.
I think the current component crisis has shown you can't trust JIT component ordering or sourcing by the PCBA company. It never was reliable, we just took the chances. So stocking BOM for the next 6 months is more important than ever.
And, I still operate by requesting quotes from PCBA companies. Right now we are using a small local one for quick turn-around. BOM optimization is as critical as it always has been.
As it stands, JLCPCB's new concept is indeed exceptional but with experience from any engineering field, it's a recipe for disaster to totally change your workflow based on a new service available from a single supplier and completely trust their workflow, however good it seems.
And I hope it all changes in the way of JLCPCB, but I don't think we are there yet.
Besides, JLCPCB is really useless for most serious builds because the component selection is so severely limited and their process won't allow sending them parts. When I think about it, I could have manufactured maybe ONE out of some dozen boards I have designed during last few years there. On the other hand, I have sent special components (not available through distributors) to PCBWay for assembly "the classic way" no problem.
And having to mentally maintain two separate workflows doesn't make a lot of sense. I do understand though that many former beginners, former "buy adafruit module" designers who would have never ordered a PCBA with the classic process are now using JLCPCB's service, mostly due to price. For them, it's a single workflow and might be a stepping stone into going "real" PCBA ordering.
BTW a 0402 is often easier to desolder for component value change than a 0805; apply a blob of solder to your typical chisel tip, touch both pads at once from the side, the 0402 part sticks to the iron tip. (Don't try to reuse the part though, obviously.)