The complete change in look from 60's TV (with randomly blinking coloured squares for buttons) to late 70's movie (with far more realistic interfaces) is subsitution, not a plausible in-universe evolution.
Nothing about Star Trek can be called "plausible" by any stretch of the word. We witness explicit in-universe evolution as the movies and spinoffs proceed, and it takes place in leaps and bounds just as great as the one from TOS to TMP. Indeed, evolution within each TV show happens much the same way. So that particular argument doesn't really convince.
Sure, the individual artists involved in making TMP might have had ideas that differed from those of the TOS makers. But the same happened again in the transition from TMP to TWoK, and things then evolved "back and forth", occasionally going closer to the TOS origins (or even beyond), occasionally trying out "contemporary" things, occasionally trying to be truly futuristic (and failing). A change in "look", no matter how thorough, is no sign of a discontinuity in Star Trek, by the standards of Star Trek.