If P2P software relied on an ISP middlebox to mediate the transfers,
then each middlebox could optimize the local situation by using a whole
smorgasbord of tools.
Are there any examples of middleware being adopted by the market? To me, it
looks like the clear trend is away from using ISP-provided applications and
services, towards pure packet pushing (cf. HTTP proxies, proprietary
information services). I'm highly sceptical that users would want to adopt
any software that ties them more to their ISP, not less.