That sort of logic can be applied to anything remotely technology-related nowadays - cars, phones, computers, etc. The question you have to ask yourself is what do you want today, and what do you need now.
I agree the USB-C thing was a bit of a letdown but in all seriousness, in the next 2 years or so, it's unlikely to make much of a difference given the limited number of accessories currently and the ones which do exist, are using older cables with the USB-C head half the time anyway which means you aren't really getting much benefit or improvement.
In other words, if it's not something you need right now, and it's unlikely that the Surface Pro (or any computing device you buy this year) will be your last, then I don't think that would be a reason to stop you from buying something which otherwise ticks all the boxes especially when you don't even know if the SP5 will definitely have USB-C. For all you know, the next Surface Dock might even come with USB-C too which in theory may be able to be used with SP4 (although you'd lose some of the whole portability benefit).
I know you said you aren't really wanting to wait so that's why I'm edging you on. If you keep waiting for something then by the time it comes, you'd have wanted something else and will end up sitting there never progressing (although that could be good if you're content now and trying to save money).