Saddle fore-aft affects pedaling mechanics and weight distribution. A 4cm change in stem length is a night and day difference in handling.
Stumbling into a frame size based on a meaningless seat tube length and then afterwards attempting to carry out fit with the aforementioned hack bandaids is dummy stupid and ass-backwards.
You are making the assumption that bicycle designers are idiots. A 43cm frame isn’t going to have a top tube from a 58cm bike. The reach doesn’t vary all that much within a brand’s frame sizes nor does it vary all that much from brand to brand. There is the caveat of mountain bike frames vs road frames where someone might try to put someone on the same frame size as they might ride in a road bike…for example, a 15” mountain bike is not the same frame size 43cm road bike…because people do that all the time.
The seat tube is the only tube that can be freely adjusted on whim. Get the correct reach frame, and afterwards you can slide the seat post any way you want. Stumble into buying the wrong reach frame and you are permanently cucked and stuck with your dummy bandaids that screw up the rest of your fit.
While the seat tube length may not match the “frame size” as well as it did in the past with horizontal top tubes, the “frame size” says that there is a certain proportion of lengths of the various geometries of the bike. Someone who is around 5’ tall isn’t going to fit on a 50mm frame although, in the past, they were presented with that frame and told to “get used to it”. Hell, at some points, the smallest frame (or the only frame) available was a 55cm (22”) bike that was called a ‘27” bike’. My wife learned to ride one even though it meant that she had to hang her leg over the top tubes at the knee and do the splits to get her foot to the ground.
At this point in time, you’d have to have a really bad salesperson to “stumble into buying the wrong…frame”.
Manufacturer's height guide is a starting suggestion at best. Two people who are the same height but have different torso/leg length ratio will need different reach frames.
If the persons in question were close to the top or bottom of a certain frame size, that could be true but, for the most part, two people of the same height are going to fit on the same frame size. Minor adjustments to the saddle and stem will fix most variances.
You need to stop screwing with people. It's bad enough that you sabotage your own cycling with your farcically wrong ignorance. Don't mess other people up too.
"Mr I don't even know what the term reach means". Dunning Kruger gold award of the 21st century.
How many bikes have you sold? How many bikes have to fit to someone?