How does a belief being typical make it any better?

If Mr Collins had never encountered another viewpoint, then that could be excused. But there seems to be a general expectation that a girl like Lydia would not be abandoned to her fate, so Mr Collins is therefore thoroughly responsible for his own ideas.

Colonel Forster comes searching for Lydia when he learns that the elopement may not lead to marriage, and so does Mr Bennet and Mr Gardiner. Surely those men are at least as typical of their time as Mr Collins?

And Mr Gardiner speaks in terms that indicate that it was taken as normal that a girl's friends would step in to protect her. He argues at one stage that Wickham should be planning to marry Lydia because:
" ... It appears to me so very unlikely that any young man should form such a design against a girl who is by no means unprotected or friendless, and who was actually staying in his colonel's family, that I am strongly inclined to hope the best. ..." (chpt 47)
This implies that Mr Gardiner not only is prepared to try to rescue Lydia himself, but that he believes that this response would be so common that Wickham would have counted on it happening.

Remember Mr Collins is advocating abandoning Lydia, not merely expressing disapproval of her behaviour. We can I think safely assume that Wickham would have abandoned Lydia when she became too much of a financial problem for him. What skills does a fifteen-year old girl like Lydia have? Unless she had the wit to turn to the Gardiners, she'd almost certainly wind up in prostitution and/or crime, with dire results from either STDs or the justice system.