A Dobbin seemed to him to be such a one as might probably be met with in the world, whereas to his thinking a Ravenswood was simply a creature of the imagination.

He would have said of such, as we would say of female faces by Raffaelle, that women would like to be like them, but are not like them.

Men might like to be like Ravenswood, and women may dream of men so formed and constituted, but such men do not exist.
Dobbins do, and therefore Thackeray chose to write of a Dobbin.
So also of the preference given to Becky Sharp and to Rawdon Crawley.
Thackeray thought that more can be done by exposing the vices than extolling the virtues of mankind.

No doubt he had a more thorough belief in the one than in the other.

The Dobbins he did encounter--seldom; the Rawdon Crawleys very often.