I'd say that you have really used that current Challenger very well to have experienced so much good time with it. It's become a part of what you are and you've probably gotten to know its every idiosyncracy - good or bad. I'd say, as would quite a few others, that you deserve to trade up after so much time with it. But let me tell you my other thoughts on this situation from my own experiences with mine (even though it's only been with me almost 4 years).
The Devils Advocate Version....
I go through that every year when the new models come out...it was particualarly hard this year with the 50th Anniversary models coming out with all the new colors and so forth. My 2016 car only has 12k miles on it and it still looks new - and even smells new inside. I will also have it paid off later this year. I also have to say that I'm not really ready to move up to a V8 model, as I'd likely not get nearly what my Lil' Twister is really worth and it is also something I have modified, from the day I bought it new, to suit me personally. Nobody else has one exactly like mine. With that being said, I can actually factually proclaim that I have a Limited Edition "One of One". Add in the fact that I still have ideas for it "modification-wise" and I can honestly say that the feeling I get when driving it is just as strong or stronger today than when I took delivery nearly 4 years ago.
First off, I can well afford a new Scat Pack financially. The thing is, if I were to get a 2020 model Scat (or even a leftover '19) with a 6.4L, I really don't think there would be such a huge increase in ownership pride and satisfaction to make that pile of money I'd shell out really worth it to me - emphasis on the me part. Sure, it would be a real tire burner with that great big, "torquee" V8 and Active Exhaust rumble, but really, at my age I'm only out to please me and nobody else. I'd rather spend the cash I have on my current car and continue doing things to it that make it a "new model" Lil Twister over and over as time goes on. The thing is more than peppy enough for the places I drive and at the same time gets outstanding fuel economy for the big, hulk of a car that it is. It has a great ride, handles nicely and is a very economical car to maintain with the 3.6 Pentastar powerplant. If I upgraded to a Scat I'd be faced with higher fuel costs, maintenance costs, insurance costs and the enhanced chance of "TICKETS" (Believe me, I know me well!). I'd also get a much stiffer, harder riding suspension, which at my age would not be very welcome at all - especially for my wife. For the younger (and some older) guys, these things may be no problem, so kudo's to any of you who desire that experience.
The bottom line, at least as I've learned to view it as my years have progressed in this life, is this...no matter how hard you try, you'll always be tempted to get the newest, latest, greatest, most lustfully beautiful thing that replaces whatever you currently drive and makes it look in some way sub-standard, and not worthy of driving anymore. My wife and I used to buy new cars and trade up every other year or so back when we were young without kids. The dealers loved us! I wish I could have some of that depreciation money that I gave away back right now. I would have been retired by now for a couple years. In comparison, you have gotten loads of value and pleasure out of your car and that's what's really cool in my opinion!
I'm sure you'll hear others give their advice and then carefully weigh-in all the opinions you get together as a whole. I just thought I'd state how I personally feel about my own situation. No matter what you decide, I'm with you though, whether we own a 2008 or all the way up to a new 2020, we all enjoy the same basic retro vehicle - Dodge Challenger!