As the ASIC miners start mining, and "outdated" miners go offline, does this not pose a risk to the network in the sense that there are fewer overall miners?
No.
When FPGA came out, did all the GPU miners pack up and call it quits, leaving only FPGa units? Nope. Those interested joined in with FPGA. The same will happen with ASIC. Everyone will switch to FPGA/ASIC miners. Even a casual miner could afford a Jala to tinker around with. Yes you're going to lose some people (those that only do it because they happen to have video cards). They'll either continue anyway (because their parents pay electricity, and they don't care about mining at a loss), or they'll drop out. Either way, they constitute a very small and rather insignificant portion of the pool. In terms of security, having 100k users (95% of which are super casual and only contribute 5% of the total hash rate) is not that mscurried secure than 5k users with even distribution of power. The security comes from total network hash distribution, not total distribution of participants.