> Any given interface is inherently rate limited.
> When demand exceeds the capacity, something must be
> done. Often this is done w/ "striping" or "muxing"
> where multiple "low-speed" channels are "bonded" into
> a single virtual path. L1 is not that different than
> L2 & L3 in these cases. The specific dynamics are
> unique per layer but the problem remains the same.

When l1 is p2p, you can do nothing at l2, unless you additionally
introduce l2 switches which are as expensive as and often slower
(as is the case with ATM) than l3 routers.

  when L1 is p2p (generally true for most deployed L1 technologies)
  you still need something to originate/terminate the information
  transmission. and when the offered load exceeds the L1 capabilities,
  you are stuck. Striping or changing L1 media are the primary options.
   Your other points wrt introduction of more stuff are true.

But, the point here is that it is not or will not be necessary to
share a single "high-speed" channel for "low-speed" pathes
to multiple ISPs through L2 switches.

  Thats true today and has been for several decades.
  But it might be desirable, for any number of reasons.