There is a lot of differences, especially when it comes down to SSD/RAM due to bulk pricing dropping off with a bigger production estimate.
It can also mean a higher end APU when it comes to both Ryzen/Navi tech due to cheaper prices.. An extra year would also Cerny and it's software team to develop better dev tools and API upgrades.
A lot can happen in a year especially between 2019/2021 (2021 will be a drastic change when it comes to the GPU market and you can quote me on this for all you want because we are about to get Intel as a major GPU player in that year)
The thing is though, if the benefits of waiting until 2020 all boil down to 'things would be cheaper', the question is, would they be cheap enough to offset the year's worth of lost sales? I think it could just as easily be better to sink more money into a 2019 machine to get those more powerful specs, launch earlier to monopolise mindshare and ecosystem retention, and use that to offset the bigger loss they'd need to take. Then they get greater customer feedback over 2020 that lets them improve the PS5, so that by Holiday 2020 is a better system than what would've launched at that time. All speculation obviously, but if the benefits all come down to money...
Agreed about 2021 though, we should be getting AMD's Nextgen GPU by then as well, surely? And hopefully ray-tracing tech and DLSS style stuff is in most GPUs and games by then.