Sounds like a good idea. Now let's see what the enforcement looks like.
I am still out of the "political" talk, but there is significant reasoning why this is not a good idea.
I liken it to tenure. On the face, tenure for faculty seems like a bad idea. They bust their asses, finally prove themselves with enough grants and research and whatnot until they have tenure, and then they coast til they die, sucking money out of the University. So, why have tenure? What happens if you don't? You make a tough choice easy. If you are someone with your phD and are looking to either go into the private sector or teach at an institution, what will make you the most money. This is capitalism we are talking about and money RULES all. Well, the private sector ends up being much more appealing since there isn't much money in teaching and no security. So, now we have tenure. Once you've been accepted by the old boys club, you are a member for life. In some ways it's a negative, but in others, it makes sure a lot of the brightest and cutting edge faculty members will come to a university to teach and have research. They know that they can be set for life.
How does this equate to lobbyists? Many government positions aren't high paying, but they are an "in" and give you knowledge of the system. You take them knowing that once you come out, you can make money by already being a part of the system. Money makes the world go round, right? So, many of the brightest and hardest working people in Washington take these government positions knowing that being a lobbyist or something similar is available to them, setting their future up. Now, a 5 year ban makes them think, ok, what do I do now? Why would I take this government position if I can't parlay that into money down the road? You now lose a lot of the brightest and hard working government employees because now you have hamstrung their earnings potential.
Maybe that is what Trump, or many other people want. Maybe it will work. The problem is that we don't know what will happen and often the unknown of a situation is worse than a bad situation that you know and can try to control (sort of like ACA). Something that seems broken may in fact be broken, but it may be a lot less broken than the alternative.