Todays external SSDs are still way slower than unified memory. I originally thought that back in 2012 with a 8 GB/256 GB 15” retina MBP, but after reading up on this and looking at various tests, I brought that model back, and got 16GB/256 GB model. It worked for 9 years, but the MacOS kept getting more complicated and used more memory, and the discrete GPU performance was something that only made things worse over time. When using 8GB only it becomes an issue of many processes being able to use shared RAM and be slowed down when they all want to use RAM instead of storage. Even back in 2012, we knew that you were slowing down your laptop considerably playing games on a Mac versus the 16GB models.
Fast forward to 2022 and AS macs, while the SoC and unified memory speed up everything to a lot more acceptable to use with 8GB, you still can easily find examples of drastic speed reductions with Mac software usage where using more RAM eliminates that. Playing complex graphical games as an example can really show how it’s preferable to load more parts of the application into faster RAM.