A plus for Software Boutique was that it was quality assured by the Ubuntu MATE Team (at the time) before each release. Everything in there was designed to present the "best in class" - especially users coming from elsewhere who might be looking for alternates.
I suppose users may not realise it never was a "full" software centre, but I guess if it existed today, it could plug in to the Ubuntu repositories (apt/deb), Snaps, maybe even Flatpaks...? (There was friction with that last one)
Code wise, it was not the best - me and @Wimpy would describe it as a "prototype gone production" - but it worked! There had been at least 2 attempts to reboot the project (years ago) but it is quite a task and never materialised. I didn't really like how snap distribution slowed the initial launch - but being a snap flexed some rules around its inclusion as a default app, because official flavours have rules of what can be preinstalled, especially if it uses third party repositories like it did.