Storage and RAM not being user upgradable is an environmental nightmare for sustainability.
Not having internal slots for storage and relying on USB or NAS is not an appropriate alternative for professionals regardless of what their leadership says is what professionals want.
We’ll never know, but RAM being part of the SoC is probably contributing substantially to their performance capabilities compared to competition. The only real way to know that probably requires being an engineer at Apple. I’d wager $3.50 that they’d get a substantial performance deficit from switching to DIMMs, and that terrifies them since that would further push everyone to x86 workstations.