Well, the Russian Army did what you proposed, with the Armata series. It's just that it didn't work out (at least not initially). as a stop-gap measure they then adopted the T-72B3 in recognition that the problems with the Armata platform are so big, they don't hope to see it resoved anytime soon. Also, the T-72B3 upgrade seems to be no worse and less costly than T-90. It's just that "better than original T-72B" turned out to still not be good enough. Whether this is a result of endemic corruption in the Russian military procurement system or, my impression, that the design flaws of the carousel autoloader are essentially unfixable, remains an open question.
At the end of the day, Russia is an economic dwarf with a military far bigger than it can sustain, so we're seeing those spectacular programs such as the Armata, but only in homoeopathic doses. At least, that's my impression.