There is still value in the MBP 2015. It just depends on your needs and expectations.
The MBP 2018 has its advantages for sure, processing power, the newest so longer supported, cool-modern design, display, USB-C thunderbolt expansion...
I recently was in the 'what to buy' mind, and it came ultimately to two major factors, with a handful of smaller. The main factors were processing power and cost. The MBP 2018 wins hands down for processing power and if that is the main concern the discussion ends there. The MBP 2018 is about twice the price (good deal, discounted price, what used 2015 is on the market - averaged out in general), these were the factors of my decision.
Then there are the 'smaller factors.' The MBP 2015 is a proven design with few issues over the years - dependable. The only major problem being the 'staingate' display film lamination, and APPLE has a 4-year replacement program from the time of original purchase. The display is very good, still one of the best. Ports that can do just about anything without too many extras (isn't that what portable is about? not having to carry too much extra). SD card and USB are very good to have available as on-the-computer ports. I write, and the keyboard is very nice, one of the things I've read about this keyboard before, is that you never really heard anything about it [before the new butterfly in comparison], and that was because it just worked without fanfare or issues, which is a good thing. Mag-safe is a truly great feature they should have continued. The trackpad is not huge, but it does the job well and with all the gestures. Speakers are not as good as 2018 for sure, but when I'm out and about they do fine, headphones for fidelity, or on the desk with the SoundSticks. It's still a good looking design over-all, not modern slim-compact, but attractive. For basic Office, and most programs it does the job very well, I may wait for video and photo rendering, but it's not a priority (personal, not for work) or done so often (weekly, not daily) that it makes a huge difference in my workflow.
MBP 2018 from the specs. is just about everything I could ask for; processing power, graphics, gorgeous and bright display, 4 full power modern USB-C ports for the future, in a very beautiful and very thin-compact design form. Money no object, it is the way to go for most buyers. It has enough of everything to be the perfect workstation at home/office/on the go. Speaking for myself, I tried the keyboard out for a couple hours and may or may not get used to it [why should I have to get used to it?]. Ports with dongles is a compromise or ideally no compromise, till the future is all Thunderbolt 3, well... it is. For work and home I still need those older ports now. Then there is that 'dependability' thing. It may not be a problem for many, but just too many [for me] reported potential issues with; keyboard failures, T2 chips (cracking speaker/recording pops, kernel panic/bridge OS shutdowns, screen connectors... As I said, the MBP 2018 look great on paper, but it's too much of a roll of the dice for me, and then there is that price thing.
So new, 2018 modern design and better processing power at the risk of dependability, and at a higher price, or MBP 2015 with lower power needs and much lower price (used), and still a nice form factor and design. Some could dismiss the MBP 2015 as just being too old, and no longer relevant. Yes, they all get 'old,' but if it's still usable and does the job intended...
For me, it was mostly the price-thing, had a new MBP 2018 13" upgraded with 16/512 on order but canceled. I got a like-new used boxed and all MBP 2015 15", made this last year (and still some factory warranty on it), fast enough with a 2.8GHz, 16GB, and will upgrade the SSD without an 'APPLE tax.' At less than half the price of a base new MBP 2018 15" or BTO 13" I wanted, I couldn't be happier with this decision. Its 'enough with more' for me.