True, but only to a point. Depth is dependent on your forward speed as well as the resistance of your lure to the water. So if you're trolling umbrellas your sink rate is going to be much different than if you're trolling a single spoon or a lure.
(Yes, the same can be said of downriggers, but to a much lesser extent.)
If you troll @ say 1 mph and you have out 150' of wire, you will be @
X depth. If you increase your speed to 2 mph your lure will be @
Y depth. You have no idea what that depth really is because things like current and wind can easily change your speed without you even noticing. Since you cannot see your lures or your umbrellas on your fishfinder (because they are so far back) you're just guessing at how deep they actually are. With downriggers, they are not as subject to the ballooning of the line like wire is. I can see the downrigger weights on my fishfinder so it's easy to accurately adjust my depth if I see a school of fish at 10 feet or 50 feet.
And, as a further inducement to using downriggers, unlike wire, I never have to worry about putting a kink in my wire that all but eliminates it's strength.
Please don't misunderstand me. To each his own. If you like wire then by all means use it. All I'm saying is that,
for me, downriggers are the way to go.