@Ryuuji_Gremory well, the lovers were at least well to do, most likely lesser nobility, and no one cared about abandoning Alejandro in part because of the bullshit of noble politics. After all he was a black mark for being cursed and unlikely to be inheriting the dukedom because he was going to be killed by said curse so he was less than worthless in high society. Now he's no longer a literal leper and inheriting the title of Duke, the highest nonroyal title.
As far as the servants, they did the minimum that was required of them, which was to do enough that it didn't look like the parents intentionally let him die.
Also, they technically didn't abandon him to die. He had doctors, expensive medicine, and in the original timeline they straight kidnapped a girl who might know how to read the curse in an attempt to cure it. As a potential heir he wasn't ignored, as a child he was neglected.
@Misticsan I've even seen that lack of social hierarchy awareness bite a handful of isekai'd characters in the bum pretty hard. It's beautiful.