They wouldn't be any longer than they are now because most gnutella clients limit the queue length to ~10.
The first-come-first-serve principle does not make file transfers more efficient so long queues are clearly not needed for a p2p network. (Rather the opposite is the case because long queues mean lots and lots of wasted bandwidth just for transmitting useless information like position in queue, next re-request time and so on.)

It may seem that way because most Gnutella users don't seem interested in large files - or rather Gnutella's selection of large files is small compared to edonkey's but gnutella can handle large files almost as well as bittorrent: The only problem with Gnutella is that most servents don't download the file chunks in random order yet, but that should change soon enough.

eDonkey is inefficient for almost any task. For downloading a single file it is very slow, the long queues and the source-exchange produce lots of unnecessary ovehead and it heavily depends on servers.
If you want to download a single unique file, from a single host, its long queues can be very frustrating unless this host has a very high uptime.

I think you can try to encourage users to share by showing them some "please share" messages. It is impossible, however, to force users to share in an open network, - or even create an efficient rating system that effectively disadvantages freeloaders (see eMule).