I think so, but even a mixing of 1st and 3rd party support isn't the biggest reason by themselves
The biggest reason imo is the fact it's hybrid; You can play AAA games in your bed, and in a train, in a queue, it's awesome
But of course, it would be meaningless if you don't have such games, which happens to be both 1st and 3rd party
So I'd say there isn't only one big factor to this, each small "factor" is feeding other another. If you remove a key aspect of Switch success everything would fall apart







