Bitcoins fixed supply is not the only thing that gives it the strength it has enjoyed thus. The fact that it inventor is also unknown and that it can't be linked to any known person or group of persons further strengthen and increase the trust that people have about bitcoin.
What makes bitcoin stand out is that it was not created as a model after any other coin. It came with it uniqueness and having stayed this long, it has become an household name for anything that comes after. They can model theirs to look like bitcoin and try to give it a fixed supply but only time will tell if it will do well in any where close to how bitcoin is currently doing.
That decentralization is definitely the reason more than anything else.
I mean a small inflation doesn't really matter to most currencies and wouldn't have mattered to bitcoin too, if it wasn't small, but like it started out at 20 million, and then grew 2% per year in supply, every year, forever, fixed rate, nothing more, just 2% more bitcoins every single year, just to promote to miners. That actually could have been better than what we have right now.
So the thing that makes it so strong, is the fact that nobody can touch it, it's decentralized, and nobody can make any changes on your account, they can't make your money worthless, they can't take it, they can't print 100 million more of it, it's just nobody is in charge, that's it.