I got my XBL subscription in Feb. 2005, so I was an early adopter. I got it at the behest of my circle of friends from back in the Halo CE LAN days, who felt playing online was far more easy and convenient than setting up a time and place to have a LAN. While I would have preferred to spend that money on gas and pizza and keep having the LANs, they all insisted on it, and it only came out to $4.16/month, so I decided to take the plunge. I had never played online before Halo 2 (I was never a PC gamer), so it never struck me as unusual or out of the ordinary for online gaming to be a premium service. I just kind of rolled with it. Here I am nearly 15 years later, still subscribed. I'm a console gamer, I still enjoy playing Halo online (and I occasionally play other games with online MP modes, though nothing that's online-only), so I continue to put up with it. I don't like it because I'm a cheap bastard, but I put up with it because I have no choice but to pay up to keep playing online. As the industry has gotten increasingly more aggressive with trying to monetize everything they can, I've been wanting proof that they need to charge for the things they deemed "premium." Did the expense of operating XBL justify a subscription fee? I guess we'll never know. At least they give you "free" games now.
It doesn't help that last generation MS really made it seem like you got what you were paying for. For the early years of last gen, PSN wasn't nearly as robust or reliable as XBL was, and Nintendo's online offerings were bare-bones basic. When you had a $50/year service that was good versus free services that were not nearly as good, it seemed like a no-brainer to go with the premium service. Sony spent last generation catching up with MS on the online front, while Nintendo never even bothered to do that much (seriously, still no proper friends list or native support for party chat?). Regardless of whether or not the expense of operating XBL justified charging people $50/year ($60/year after Nov. 1, 2010), the fact that XBL was originally an objectively superior service and the only paid online service of its kind reinforced the idea that to get good online service on a console you had to pay for it. Starting with the PS4 PSN was finally able to match XBL, including needing PS+ to be able to play online, cementing the whole "if you want a good online service on consoles you have to pay a premium" thing. Even Nintendo has a premium service that offers access to free NES & SNES games, though their online is still a joke (trying to play Mario Kart 8 DX with a friend was a huge hassle).