Except most of Simnel and Warbeck's support in OTL came from Richard III's relatives and allies.
Well, as the young lady said "They would, wouldn't they?
Richard III's adherants were the "outs" after 1485 so they were the ones who needed a pretender. Had Richard won at Bosworth, some other faction would have needed (and found) one.
Whether you think Richard murdered the boys or not he can easily people of the time did not know and Richard can easily come up with a credible story which would defuse rumors and point to his numerous other not-dead nieces and nephews as counter-examples.
All that we do know is that within months of Richard's accession, everyone who mattered was behaving as though the Princes didn't exist any more. The rebels of 1483 declared for Henry Tudor, not for the ex-King Edward V, though many of the risings were in traditionally pro-York areas like Kent. And at Christmas 1483 Henry publicly committed himself to marrying EoY - a totally pointless gesture if her brothers were still alive. By then he was clearly confident that Richard wouldn't (or more likely couldn't) shoot him down in flames by producing any live Princes.
The reactions a decade later when Perkin Warbeck showed up would indeed suggest that the boys' fate was uncertain, but actions at the time plainly indicate what most people thought was the "default assumption" .
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