If that "disaster" was so "inevitable", it would have happened ages ago.
It's not like it was somehow possible to accidentally sideload apps. You have to first find the correct option from the system settings to enable sideloading, and then approve the specific app source you want to install from.
It is not like how things are/were on Windows. Back in the turn of the millennium, it was easier to catch malware than it was to install useful apps. For former, you only needed to double-click on an email attachment, for the latter, you needed to actively to go look for the website of the app developer, and download it from there.
Android already was pretty much at the sweet spot between security and freedom, what it came to sideloading. What Google should have done was to crack down on the scam apps in Play Store. However, they are not going to do that, since it would cut their profits.
Disasters can hapoen slowly. This one did, in a series of decisions from multiple actors. The main inflection point was allowing third parties develop for phone platforms. Then banks erc. went through a process that ended up forcing the use of a smartphone exclusively for a lot of applications that are sensitive. The same device runs random code downloaded through various means (app stores, preinstalled bloatware installing even more crap on cheap phomes, websites, embedded webviews for ads...). This is now an entrenched status quo spread across multiple actors and unaligned interests.