it makes them feel better about living mediocre lives and not accomplishing anything?

basically

I thought this was gonna be a play on Believe

But no, I don't think people who believe in an after life do it for the reason you stated. Death is a harsh reality and a scary prospect, the thought of possibly spending eternity in heaven with the loved ones you lost along the way is a comforting thought. It's more for peace of mind than anything else

I used to believe it because I was afraid to die and I needed an "excuse" to keep that fear" distracted" but... Sadly I lose that distraction mechanism and now I'm just afraid

Yep.

Well I believe that's part of the reason why the idea is so prominent in most belief systems; keep the people poor and uneducated, tell them they don't need upward mobility ("no rich man will get into heaven", or something like that), and let them believe that if they stay in their place and follow the rules until they die, they will be rewarded, despite the fact that we have no idea if anything will come to us at all.

On a personal level, I think it's more to do with wanting to see our loved ones when we die. My family has never been more religious than after my father passed, all for that hope that there will be some grand (eternal) reunion That and the fear of impermanence - the idea that we truly have a finite amount of time to accomplish our goals, and once we're gone it's GAME OVER is a scary and depressing one, especially for the vast majority of people, who are just living 9-5(just to stay alive ) and know deep down that they're not accomplishing any of what they wanted while the clock is ticking

I don't think so. I believe that it gives people a degree of comfort in the face of otherwise uncontrollable circumstances. Seeing your loved ones, having "salvation", and the promise of paradise can all be comforting to people when they are going through a rough time.

You can never attain any of the religious concepts of the afterlife if you just live a mediocre life. The standards and requirements are so high if you want to make it to heaven/achieve nirvana/escape the cycle of reincarnation,etc. that most people end up not accomplishing them all. If anything, the paths to afterlife both in Abrahamic and Dharmic religions represent a higher state of being or spiritual awakening and renewal of an individual.