Yes, what you are advocating is an intelligent choice, and it is the one that I would make, but I'm not on a crap CPU that can't do what I want it to.
If he can spend $200 now to get playable games for a couple years, vs waiting and not getting what he wants out of his machine for a year or more, then spending $350 on a new Pentium machine that may or may not perform better than the one he could have had for $200, just to have an upgrade path that doesn't make sense 6 months later because new generations of CPUs are out and might be a "better idea" and he has to buy a whole new setup anyways and is spending even more money, spending the $200 now would be better for him.
We'd need a crystal ball to give the perfect advice, but we don't have one. I'm not going to suggest "Hey wait 6 months and get a machine that doesn't perform as well as the one you could get today, just on the off chance that you might want to put a faster CPU in it a couple years". That is a sub-optimal thing to suggest in this case. If you go into a build planning an upgrade it is a good strategy, but if your reason for building is to play games now the argument falls apart. You get the best performance you can as soon as you can so that you can just play... and in this case that is a Haswell i5 and motherboard, which will be cheaper than a Coffee Lake Pentium, motherboard, and RAM, and give him better performance.