Convenience is King, and Project Sigil never gave off an air of convenience. It always seemed like whale-bait at best to me - maybe a place for dedicated hobbyists to do some neat stuff, but never something your average D&D player would spend hours in. Seems like WotC is learning that lesson now.
That's probably right but I do think they could have got it decently popular if they'd just made it so, as they suggested initially was a possibility (and the very first announcement), that you had the maps from any adventures you'd bought from WotC.
I mean, four big problems here though:
1) They'd have to create literally 10-50x as much terrain as they have in the current release of Sigil just to cover all the maps in the major WotC adventures (but they didn't have to do it all at once, it could be over time).
2) They'd have to actually build out the maps, replete with encounters, which is quite a lot of effort.
3) Sigil would have to actually work properly - IT DOES NOT right now, to be clear. For example (and this is one of many), there's fog-of-war option but all it does is put solid "fog banks" which Sigil treats as actual objects, and which cannot be seen through by anyone, including the DM who puts them there, and don't even have a state-change like doors in Sigil do.
4) They'd need a character generator that has more than just humans/elves/dwarves/gnomes/halflings/orcs in it, skin and hair colours that aren't "undyed RL Earth standard", more than one face per race (humans have three oh look out!), many different outfits, etc.
There are other issues. They could all have been overcome with proper planning and effort and money but... it would have been tens of millions to
potentially hook some people.