The truth was that Leonel wasn't the only one who felt that keeping the ecosystem of this world intact was important. Even on Earth, much of the concrete jungle had been replaced with large swaths of forest and natural ecosystems.
If they destroyed the natural environment of Earth, much of what it meant for them to come from talented worlds would be destroyed. After all, only by allowing Earth to grow in its natural state would the resources they were so looking forward to grow into maturity.
The same could be said for EarthX1. It was just that they had barely gotten Earth to its current balanced state, they hadn't had time to worry about other worlds, not to mention the fact that EarthX1 had only recently entered Earth's Fold of Reality so there wasn't anything they could have done earlier.
On such a time crunch, they could only take the most forceful approach. But, Leonel had other ideas.