I believe the reason for not flushing a transmission that hasn’t been flushed within the last 50,000 miles is that it can dislodge sediment that can then clog a fluid passage and cause the transmission to malfunction and require an overhaul. Vacuuming out the fluid may be different.
Have you considered dropping the pan every 60k miles and then adding your own drain plug so that you can just drain and refill the fluid ever 20k miles or so? I recommend removing the pan at least every 60k so that you can clean out the pand and the last bit of ATF where all the sediment resides.
Transmission fluid change horror stories may be partially be due to modern ATF being thinner than the the previous version ATF, yet the bottle still claims to be compatible with the older version. It may meet the lubrication requirements of Dexron 3 but it is still too thin! Putting thinner ATF in your transmission can cause a lot of problems in a worn out transmission even though you can get away with it if it’s in good condition. A bottle of ATF is ONLY the newest specification listed on the bottle. Dexron 4 is only Dexron 4 and not Dexron 3. If you need Dexron 3 buy only Dexron 3 and make sure it does not say Dexron 4 on the bottle. Anyway that’s where these don’t change your transmission fluid stories probably come from. I’ve done it and the transmission would no longer shift smoothly with Dexron 4.