Let's break down the points...
Why do you want to retire early?
Well, so that I can do whatever I want. Really. If I feel like doing nothing then I'll do nothing. If I want to go cycling/exercise, I'll just go. Maybe go fishing, make new friends who are already retired. Maybe some gardening, etc...
I'd be so bored. There's nothing to do.
That's cos you aren't thinking of what to do. Cos you're so used to just waking up tomorrow and going straight to work that you can't think of what else there is to do besides work. When you were younger as a kid before you were working, what were u doing when you had nothing to do? Kids never have nothing to do. They don't work. But they always had things to do. What has changed? It's cos you've gotten lazy to think of alternatives. You're so used to waking up, washing up, wearing your nice suits and going directly to work as a habit so much that if suddenly this disappeared. You would have no idea what's next. There's always something else to do. Go find it. Stop using this as a lame excuse.
My brain will degenerate. I'd want to go back to work.
Seriously? You use your brain only when you're working? Gosh... I really don't know what to say. You could read up on programming and develop apps just for the sake of it, you could grow some plants, read self help books. Plan a road trip, make cushions to sell and donate the proceeds, etc. All these use brains. They don't use the same skills as what you probably used but they use brains and skill. Your brain will degenerate only if you let it. It's the same as muscles. If you stay at home and stare into space and watch dumb shows, yes your brain will turn to mush.
Putting it all together, this comment which I get from some people is totally ridiculous. I mean, even the "I don't have enough money" comment is worth more thought than this. At least that comment potentially could be valid. If you have a large enough amount of money to generate enough income, then the comment is invalid. If you don't have much stash, the comment of "not enough money" is valid. The "nothing better to do" comment IS NEVER VALID. It's a choice and it's always up to you to do something about it rather than just, well, waking up and going directly to work, without thinking of anything else you COULD be doing. Like sunbathing at the beach.
If you really really love your job then that's all fine. But if you got enough money and don't appreciate what you're doing then, really, please just do yourself a favour and make a move. I know some people who love sales. So yea, they love their jobs, they have enough money but they just like the satisfaction of meeting people and getting sales. Same as people who love programming and being developers. Or people who love to start new businesses. You could stay and continue doing what you want. Retirement isn't for everyone. But pls don't give a lame excuse if you have a choice.
That's all it is really. Do you have a choice? Are you fully utilizing it? It's your life really. You only answer to yourself. I hope at the end you'll live a good life and I also hope that at the end, I'll maximize all the choices I have to make the best of my life.
Are you wasting your options? Letting them expire for no good reason? Many people save so that they can have the option to retire or find a better alternative although it might be lower paying. But most don't ever leave til its too late or til their old and sick. Do you know someone like that? Are YOU someone like that?
Your Life! Your Choice!