Good point. I still don’t think it’s that black and white. I think you could use either phrase and convey the same message.
The point is to form the habit. If you make it a habit to avoid prepositions, you will be less likely to make the mistake of placing a wrong one in the future.
I’m not sure what you mean here. I typed, “where are you going to?” and came up with this as the very first hit.
With respect, if you thought “this” or “it” were prepositions, you really don’t have a good idea as to what prepositions are.
And just knowing the list, by the way, doesn’t really give you a very good idea either. Many of the words on that list have a prepositional use but also can be used as part of a verb. To have a good idea what a preposition is you’d need to be able to reliably distinguish between these two types of uses.
Again: You should not have strong opinions about things you don’t understand.
You said you’re “not clueless” but to be honest, without exaggeration, I can say confidently that thinking “this” or “it” can be a preposition, and not knowing that a list like that is uninformative, all of that actually does put you pretty close to the ‘clueless’ end of the scale when it comes specifically to this particular topic. I mean I wouldn’t have normally used that word but since you put it out there… 
+1 Seriously.
I can accept that. I guess I’ll have to do some more research.
Too soon?
Who says concision is always valuable?