I think another big problem that definitely contributed and accelerated it's downfall was that Activision was making Skylanders an annual release when they shouldn't have. That's too many toys to release in that short time span for people to keep up and stay interested without it getting old and too expensive very quick.
I think the toys to Life trend could've definitely stayed longer and potentially more profitable if Skylanders came out with a new game and toys once every few years where people could keep up amd not feel the need to spend money and waste space every year on toys and feel a bit more desperate for a new Skylanders game and toys since the games would release once every few years.
But Activision milked the franchise way too fast.