They are entry level rides. The current market share is not static. The manufacturers are hoping to grow the market share (see Honda Grom), not just steal a piece of the current pie, ie get more people riding. Entry level rides have lower insurance now, fact.
More riders means more insurance customers. There isn't a limited amount of insurance available, so increased insurance demand will not necessarily lead to higher prices (partly because a larger insurance pool will diversify the risk). From the insurers perspective, 2 people paying $700 a year on their 'safer' 250s (what I paid at the start) is better than 1 person on a 600+ paying 1400 (what I pay now, and I have one write off in my bag too). Basically, all else equal, more is better for insurance. For now, insurance price and lack of accessibility keeps people out at the margins. Expanding the margins brings more people and theoretically better insurance. But who knows, maybe insurers won't see it that way...
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