Records aren't a fair comparison - they're too rider dependent. Yes, in a combined 250/300 class the 250 record holders would likely win. The people that would be affected are the riders of equal skill on different machinery. When a 300 ends up beating a 250 because of machinery, it will become less fun for the 250 rider. That 250 rider might even want to spend more money to build a 300. Riders looking to enter that class might not want to buy the more-affordable, less competitive 250s, so they either spend more money building a 300 or, worse, don't enter the grid at all. The healthy 250 grid begins to disappear and the 300s continue to slowly grow (as they would grow anyway).

The 300 class is a slowly growing class, but handing out less trophies doesn't make cannibalizing the 250 class a good idea.