I can't give you a definitive recommendation, but I'm in a similar spot as you, both because I'm thinking seriously about buying a new 16" MBP w/64GB RAM, and also because I'm a Java/IntelliJ developer. Some random thoughts on both sides of the argument:
- The biggest thing to worry about is whether you might be underspec-ing a machine, such that it won't be capable of performing adequately with whatever the latest version of OSX will be a few years from now. The good news here is that the lowest-spec'd machine that Apple still sells brand-new as of today is a MacBook Air w/8GB or RAM and a 128GB SSD. They aren't going to leave those users in the cold in just a few years, so they're kind of limiting themselves to keep the OS storage and RAM requirements within those limits for at least a 4 years, IMO.
- Adding to that last point, if you can comfortably run their OS today with just 8GB of RAM and a 128GB SSD, think about how much more headroom you'll have with 16GB or 32GB of RAM, and a larger capacity SSD.
- From the opposite side of the argument, I today have a late-2013 MacBook Pro 13" w/8GB of RAM and a 256GB SSD, and I'm close to the storage limit of my SSD and am often running close to my 8GB RAM limit, even just browsing with the web (with, admittedly, a lot of tabs open). So, I definitely feel some pain today, and if I'm going to spend some money to upgrade, do I *just* want to double my RAM and storage, or should I pay even more to go beyond that?
- As I just noted, my current MacBook is a late-2013 model, so about 6 years old. If I can stretch out a MacBook to last me 6 years, and it costs me an extra $360-400 to go from 32GB to 64GB of RAM, that's about $60/year. Tacking on an extra $360 today might feel a little painful when you're configuring that $2200-$3000 MBP 16", but if you think you can keep it for 4-6 years, that extra amount to upgrade the RAM (or SSD, or CPU, or whatever) doesn't seem so awful.
- I haven't run the numbers on this idea myself (yet), but maybe instead of trying to stretch my MacBook to last me 6 years, maybe I should do the opposite and buy the lowest-config'd MBP 16" and plan to sell it (or trade it in) in 2 years, and get a brand-new MBP every 2 years. Like I said, I haven't (yet) run the numbers on this idea, but again, if you figure out what the "loss" is per year, maybe it's not that awful. That said, I think I'd be unhappy with just 16GB of RAM doing software development, and would want to at least upgrade to 32GB.