Someone can get quite a bit of power after 6-12 years in the Congress or Senate. Why do they stick around for 30+ if it's so easy to cash in on their contacts and so forth? They stick around for the power to make policy, not for the chance at future riches.
Chris Dodd makes a decent living heading the MPAA - $1.5M/yr according to Wikipedia. While he was in the Senate he probably averaged 10% of that. He's not going to make up his "lost" earnings in his remaining lifetime - he could have made a lot more than $150k a year during those 30 years.
Lobbyists give donations because they want government contracts and favorable treatment in the law. Since we don't have a reasonable public financing system for public office, and since mass-market advertizing is so expensive, and since the "franking" budgets are relatively tiny (and not available at all to challengers), public officeholders are receptive (or at least give the appearance of being receptive) to lobbyists arguments. But they wouldn't vote that way (on the whole) unless they already had that inclination.
You think money or a promise of a good job would get McConnell or Whitehouse to change their minds on a vote? I don't. They'll be fine after they leave office no matter what the lobbyists want and they're not motivated by the desire for riches.
My $0.02.
Cheers,
Scott.