I agree. 2 is too little for the legitimate test sides. But even 2 tests against Zimbabwe is painful for everyone.Not practical and not advisable.
There are some cases where 3-Test-series are enough. 2, no, 3, yes.
Of course, there is the occasional series that merits 5 games - India-Pakistan, for instance.
What I would like to see is the series length adjusted based on current world rankings. Like WI and Eng play just 3 this year because WI is in the dumps. Similarly, SL should start playing some 4 match series.