Mavs fan here and I commiserate. Kristaps obviously brings different things to the table than Gobert (better shooter/range, not nearly as good defensively), but the underlying issue is the same.

For a long time, the ideal was to have a big man who could shoot because it makes the opponent play a smaller, nimbler defender to hang with him and reduces the effect of the lumbering big man of yesteryear.

The issue comes when you have someone like Porzingis who can shoot, so the opponent goes small to counter it and then he can't make them pay in the post. In that case you're no longer a player who can generate mismatches, you're just a shooter who happens to be tall. Even something like that though can take Gobert's effectiveness off the table. Gobert is elite, but he can be run off the floor defensively and doesn't have the offensive chops to make it hurt more of the other side.

If you play a unique or different style (Porzingis, Gobert, Simmons) you have to have the versatility to still be able to do damage when an opponent modifies lineups/strategy to take away your singular strength.