Comparing MASH finale viewership with today shows' viewership is comparing apples to oranges. Until late 1970s or early 1980s, each episode was to be watched in real time, because the only alternative was to spend years waiting for rerun. Furthermore, choice of what to watch was very limited (only three major networks in USA and even fewer in other countries).
Numbers reached by MASH became impossible to reach for every show produced in time when VCRs (which allowed for later viewing) became widely available.