I've been reading a few other questions and they got me thinking.
So Muggle-born wizards get a visit from someone from Hogwarts, others (wizard-born) get a standard letter.
Dear Mr. Potter,
We are pleased to inform you that you have a place at Hogwarts School of Witchcraft and Wizardry. Please find enclosed a list of all necessary books and equipment. Term begins on 1 September. We await your owl by no later than 31 July.
In this question, it seems to imply that if you don't respond to the letter, you might not get to go to Hogwarts. It also (in the comments) suggests that Hagrid was only sent because Harry had not read the letter (somehow they know this).
So the question is simple. If the Dursleys just gave Harry the letter and then laughed at him for being gullible (no need for Hagrid to visit as he read it), would that mean he wouldn't be accepted? (Ideal solution for the Dursleys and a lot easier than running off to an island).
Also related, is there any evidence what happens if you don't reply?