My response is kind of like reznikoff's. This sort of daydreaming started for me when I was young (elementary school) but it became much stronger in middle and high school - possibly because I discovered online roleplaying along with a couple of other friends, so we indulged each other. Towards the end of high school, a lot of my friends had moved on from roleplaying or had much less time for it. But I wasn't ready to move on at that point. I'd grown up in an abusive, poor household and did not have any hopeful future prospects. Like many high school friendships, some of mine ended in fights or conflict as people went bonkers with hormones or their personality changed. Despite having a couple of friends after graduating I still felt lonely, rejected, and hopeless. So I cut my ties with people and retreated into my own little world. And it ... brought me nothing in the long run. A couple of years ago I realized that I had done nothing with my life since graduating and that my fantasy world(s) were consuming me, making my real life much harder - I wasn't making friends, I wasn't accomplishing anything, I was poorer than ever, and I was still lonely and wanted to die.
So I made myself stop being so involved with it. I forced myself to abandon that inner world and reconnect with a friend I knew and liked, and that went well (we're still friends, and we both struggle with depression and high functioning autism so she understands). I applied to college (not going so well ...). And I started trying to actually lead a healthy life, even if it wouldn't ultimately cure my depression/suicidal ideation.
I hadn't known how bad maladaptive daydreaming could be until I met my last girlfriend (a relationship that ended a few months ago). We met through literate roleplaying online (I was starting to try and take writing up again but felt lonely with the hobby as well as rusty) and clicked. However almost immediately I noticed that she was as obsessed with her own little world as I had been with mine years ago. Every topic of conversation we had always somehow wound back up with her talking about her characters and her persona, even if it was completely irrelevant. Any time I or a mutual friend went through a hard time she didn't seem to care. A friend of ours had a close friend die and wanted to talk about it and her response was "Oh well, death happens, what are you going to do about it. Now, do you guys want to hear about _ that I created/thought of for Evermore (her imaginary world)?" She had no sympathy for anyone else but felt personally wounded whenever someone didn't pay obsessive attention to her story and characters, to the extent that she would spend literal hours whining about how hurtful it was to her when others criticized her work or even vaguely disliked it. It was like her only interest. And then she created a fake social media account to spy on myself and others and try to impose her fantasies on real life, got caught, and promptly blocked all of us.
It was around then that I began to feel and realize how absolutely pathetic we both were at different points in our lives. "Maladaptive" daydreaming is the perfect term for it because it's true. Focusing all your time and attention on a fantasy world to such a degree is escapism and it WILL eventually become your coping method if you let it. Spending your days dreaming of a fantasy world that will never, ever exist to the extent you abandon your actual life and hurt other ACTUAL people to maintain that for yourself ... well, it's sad. And frankly if you're going to live your life entirely in your own head to the extent that you don't give a shit about anyone else who actually EXISTS, then maybe do the rest of the world a favor and do it alone rather than dragging others down with you.
None of this is aimed at anyone specific of course, just figured I'd share the dangers of doing what OP mentioned. Another friend I had a few years back, an older mentor type, had a girlfriend who tried to literally become a character from a D&D campaign they had that became long-running because she also became obsessed with the fantasy. So as far as I'm concerned what I experienced is not an isolated incident either.
