Sure those big routers are costly but they're maybe the cost of a dozen satellites and serve half the total constellation size. Buried infrastructure has a 20-30 year guaranteed lifespan if built with fibre. Routers tend to last 10 years minimum. The opex cost of constantly replacing satellites is going to be pretty substantial.

Also Starlink doesn't have last mile costs as such (they just push that to the consumer) but they do have ground station requirements which will grow linearly with demand. It'll be interesting to see how they manage that as they grow. And of course, they're going to need those big routers too...