Very true.
Less true. Most abolitionists came to it from a moral place. You can actually see this in, for example, boycotts on goods made with slave labor, such as sugar. This movement began in the US with Quakers, but spread quickly to many others as well. It never got super popular, but that was mostly from laziness on the part of consumers - this was before sugar beets, so boycotting slave goods essentially meant no sugar, as well as no cotton clothing, and no tobacco. Now, it's true that wanting blacks to be free wasn't the same as wanting blacks to be free...and nearby, but to dismiss abolitionist sentiment as being entirely or even primarily economically motivated is just wrong. Even sending free blacks "back to Africa" was often from a place of genuinely wanting to return a people to their home...though here I'll grant that a lot of it was basically a way to get rid of blacks.
I've studied the Northern abolitionist movement in some detail. Basically, there were three main arguments against slavery in this time period:
1 - That slavery is morally wrong.
2 - That slavery offers unfair economic competition for free laborers.
3 - That slavery means having black people in the country (the horrors!).
#1 is the most appealing argument to us today and it had some adherents at the time, but not nearly as many as people now want to believe.
That's why the boycotts failed; most people really didn't care that much about how much Caribbean slaves suffered.
The reality is that #2 and #3 had broad acceptance. If you read a lot of "Free soil" arguments, they can be appallingly racist. "Free", in a lot of peoples' minds, meant being free of black people altogether.