Not a week goes by where I don't see a remark somewhere on some forum that the current trend of soloing is ruining MMO's. But I think they have it all wrong. In comparison to older titles, the more recent ones are actually less solo friendly.
Let's look at EQ1, what many consider to have been the ultimate grouping game. A good portion of folks I knew there soloed at some point. Every one of my alts soloed, of course some better than others. It took a long while to kill something, your experience bar didn't move much, and maybe you had to stick to green/light blue mobs. But it was done. Typically, I'd go to a zone and start soloing. If someone shouted out that they were looking for more, I joined in. If someone shouted looking for a group, I invited them to join me and eventually more would join. While leveling was still slow, grouping took some of the tedium away and just made things more enjoyable overall.
In more recent titles, leveling speed is increased and quests often yield more than the kills. It's not efficient to group since many are on different quests. And forming up groups is more tedious than just going out and getting it done on your own.
All that's changed is leveling speed, color of mobs you can take on, and the introduction of quests. But there's one major difference in current titles from prior. There is no meaningful solo content anymore. In EQ1 there were numerous dungeons that could be soloed, many with placeholders for named mobs that dropped nice items. This wasn't exclusive to dungeons though. Named mobs could be found in outdoor zones as well, if you didn't mind camping a spot to get them to spawn. In DAOC also dungeons and named mobs indoors and out could be soloed, both for armor and trophy heads which could be mounted and placed in your home. I didn't have the best gear and didn't always have the best solo class. But there were meaningful things for me to do on my own while out adventuring. Even SWG was more solo friendly pre-NGE than it is today.
In the current games I've played, you solo strictly to level. It doesn't lead to eventual groups and usually gains you nothing but experience. Anything meaningful, and what I consider to be fun, requires groups, usually full ones at that. And they are organized events, not chance encounters while you're out and about. Occasionally you might meet someone on the same quest. You group up for a few minutes to get it done and then part ways. So come to think of it, not only are current games less solo friendly, but also less group friendly.
So soloing has always been around. It's just not very fun anymore.