As for keeping 4G speeds up long term, I figure...
1) more spectrum. VZW has 700mhz spectrum, as well as AWS (1700/2100mhz) spectrum for LTE, and long term (once enough users have LTE phones) they could reuse some of the 800/1900mhz spectrum for LTE. This alone will help a lot, they've really got quite a lot of 700 and AWS. Technically EVDO could run on these I suppose but it's not (well, other than MetroPCS, who DOES run CDMA and/or EVDO on AWS.)
2) Faster speeds get each individual user off the channel quicker. Pre-streaming some slacker radio, or a youtube video, will be almost instant at LTE speeds, versus taking several seconds on EVDO. Meaning, in aggregate, you'll be sharing the channel with fewer users.
3) As for the heavy user, I read a few years ago when Japanese ISPs began running 100mbps service, contrary to (US ISP especially) conventional wisdom that heavy users will max out any connection they get 24/7, they found even the heaviest users were having their bittorrents and junk finish faster than they could come up with new ones. I suspect this could be the case with LTE as well.
4) Network management. If all else fails, hopefully they'll get over these capped plans, but network management does help. What the satellite ISPs do is have unlimited usage, but they have a MB bucket, say 500MB, when that MB bucket empties the user is throttled. If the user quits using data, the bucket refills at the throttled rate. Overnight, since usage is light, the MBs are not measured and no throttling occurs. Users like in #3 could easily be kept from impacting everyone else either through a thorttle, or giving "lowest priority" network access until they lighten up.