This is very interesting and I'm happy to not be alone to wish it.
The landscape will never be completely covered, because plants stops spreading when there are 4 plants in a radius of 4 blocks.
It's possible to change this. I've already thought about it but dropped my project, because of mods compatibility problems.
The idea is to remove plants with a small probability. Plants spreading uses an ABM (Active Block Modifier), a program which is run for every concerned block, with an adjustable interval and probability (in this case it's run every 50 seconds, with 1 chance over 25 for every plant block). This program (see
here) checks all of the conditions (e.g. lighting or plants density) and duplicate the plant. We can easily add a probability to remove the plant in this program.
If you know a little LUA language and Minetest technical features, it's not very difficult.
But I want to rewrite it completely, because it's not really well made, I want to add a way to manage differents plants with differents characteristics.
I will perhaps make a subgame, one day. In this case I will include this improvement.
For trees, it's a little more difficult.
In Minetest, trees are completely inert. They don't spawn naturally after the map generation, except if you plant a sapling, and they never disappear. So to implement this feature, we must code it from zero. And it's more difficult, because a tree is not one block but a lot.
It would be a great enhancement.
Very busy this year too, so do not expect me to be very active on the forum or in game. But I'm not about to drop Minetest forever :)