In Act 1 here, you are barely scratching the surface of the underdark. In 5e, you can pretty much throw your players into any environment and modify the creatures there (there are rules for this in the DM's guide and other sources), so that your players have a chance at survival. So far, I'm enjoying the game, and can't wait to get to other areas.
In some settings you don't even need to step into the Underdark. Mindflayers, Drows and other things will attack and raid the surface...
True ...
But not this one ... at least not yet, who knows what Larian prepared for another Acts.

You know im kinda sad about our chances to meet any "low threat" enemies ... in future. :-/
For example i especialy like kobolds, but what are chances we will meet them, now when we allready fighted Duegar slavers, Minotaurs, or Githyanki?

I wouldn't be surprised if we did meet them. Look at the goblin camp. We may end up stumbling into an entire city of kobolds. Even if they are low CR, enough of a mob and it becomes extremely challenging, and interesting when you have to deal with the numbers. :P