In my entirely casual understanding of English most means the set that has more members than any other. When the comparison is binary (sites that work vs sites that don't) then "more than half" is both necessary and sufficient as a definition.
When comparing more than two options most could be significantly less than half (e.g. if I have two red balls, and one ball each of blue, purple, green, orange, pink, and yellow, then the color I have the most of is red, despite representing only one quarter of the total balls.)
That said, any attribute attaining more than half of the pie must be most.
In retrospect, the never in my previous comment was certainly an overstatement. While I agree with your reasoning, there is often a distinction between technically correct use of language, and what the hearer is likely to understand from what is said.
> About 25% of sites become unusable
These two statements seem pretty contradictory. 75% feels like a low threshold for "most."