Struggling with the ethics of letting my cats outside

I know this is an age-old topic, but I need some advice in regards to letting cats outside and whether or not its OK to do so (jump to the last few sentences if you want to skip the excessive context). I have two cats right now and both of them live indoors but have free access to leave the house during the day to play outside. They LOVE being outside; they spend all day exploring, playing with bugs, chasing eachother, climbing trees, lounging and rolling in the grass, etc. We live in a urban-ish suburb in Colorado.

My dilemma is that I am constantly worried about one of them getting hurt or killed by humans/cars/racoons/etc. I get stuck on the fence when deciding if I should convert them to full time indoor cats. When I think too far into it, I theorize that quality of life can be determined by the relationship between exciting experiences and safety/well being. I can let them live their lives to the fullest at a higher risk, or I can let them live their lives the longest with significantly less excercise and stimulation. I always roughly settle on the decision to keep things the way they are, as they currently get the security of a safe shelter and a good food source, but they still get the freedoms of going outdoors during the day. I still feel so irresponsible though, especially when I see people talk about it online (they always say its very bad to let them outside). But then I see how happy they are while playing outside, and I feel more pain at the thought of taking such joy away from them. I am currently in college and cannot spend much time with them during the day like I used to, but I still want them to get lots of stimulation and have some choice in the way they live their lives.

My parents held me back from getting a motorcycle as long as they could when I was younger, but then I turned 18 and had the cash to get one. I have been riding my little Harley sportster for a few years now, and it has changed my life in so many wonderful ways (even though I know it is dangerous). Driving a car is also dangerous to an extent, though. If I ever got rear-ended by a semi on my way to school, I would much rather know that I died having gotten to experience the motorcycle instead of avoiding it all these years to be safe. There's just no feeling like it, not to mention all the close friends I have made and fun trips I have gone on because of it.

I am thinking about all this because my oldest cat is turning 8 soon and he has been outdoors all his life. I see him starting to get older though; I worry that his chances of staying safe are dwindling and I just don't want to lose him. I don't worry too much because he's a total bad*ss and he also gets moody when I make him stay inside, but he's not a kitten anymore and I know that he has to start declining at some point.

If you happen to see this post and read my dumb rant, what is your opinion? Do you have cats, and what do you do with yours? We all have one life that we get to live, how do YOU draw the line between maintaining the length of your existence but also experiencing it to the fullest?