While that may be true about platforms, it's not that different from BF. The important thing is the slants. Very few characters can actually fight efficiently on tilts/slants (MK/Marth being two of the most proficient at it). The slants often change the spacing needed for moves for them to land. Take the fact that most characters can't fight on slants well, add in MK's great ability to, and you have MK getting a huge boon from the stage.
For a very simple example to explain this:
Lylat is tilting to the left (left lower than the right) with MK being on the right. In order for anyone to come at MK, they have to approach from a downward angle, very similar to as if they were to approach you from the air. SL, Ftilt, and Nado all become amazing anti-approach tools since they're functioning as AA's as well as normal stuffing approaches.
Similar things happen when going the other way, since not many characters have AA's that are good enough on their own to function as ground stuffing options as well (Marth has Jab and Ftilt, most characters have... not a lot). MK then has this benefit and further aids to his dair camping since it exaggerates that same angle that makes MK's dair camping difficult to beat for many characters in the first place.
Agreed on people who aren't used to the stage have trouble recovering as is, MK's offstage exaggerates that problem as well.
Etc etc