First of all, as other have said, adding FP to an attraction either legacy or FP+ will increase the standby line. The trouble is that's not the question we should be asking. The question we should be asking is does the typical guest perceive their waits in line as being long or short amongst all of the rides. That involves combining their waits in both SB and FP lines. The trouble is, it doesn't need to be reality it only need to be perception.
For example, if I gave you the choice of 2 touring plans for riding 12 rides. In plan A, every ride had a 10 minute wait. In plan B 11 of the rides would have a 3 minute wait but the 12 ride would have an hour wait. Which plan would you choose. A large number of people would want to avoid the hour wait and pick A. When in reality Plan A has 120 minutes in line and plan B has 93 minutes in line. And, in term of perception, plan A may in fact feel better even though you spent more time in line.
IF FP+ has lead to greater utilization of the FP system over legacy FP in terms of numbers of people using it for whatever reason: Not needing to run all over the park, getting 3 at one time, prebooking, increased advertising, perceived need to prebook "headliners" etc. than more people are avoiding the some of their long wait. So on the whole people may be happier even though they are waiting in line longer for everything else.
For example, let's assume a busy day at EPCOT. Under FP+ TT and Soarin' are completely booked up by shortly after opening. Under legacy people would contend they could easily get multiple FP's for both. One could conclude (assuming no computer glitches) because of tiering, under FP+ more unique guests are getting to skip the SB on TT or Soarin' where previously they may have stood in line in the SB line for both because of perceived inconveniences in legacy FP. Also because of FP+, Nemo, Figment, and Spaceship Earth have a 10-15 minute wait where previously they were 3-5 minutes. But you've got a FP for one of those as well. So before it was 60+60+3+3+3=129 minutes in line. Now it's 60+10(FP return for headliner) + 15 + 15 +2 = 92 minutes in line. The person who previously didn't use FP is happy. Yes the FP expert who could pull both and not run themselves ragged is unhappy. They were waiting 10+10+3+3+3= 23 minutes. But the only reason they could do that is because other people thought legacy FP was impractical for their plans compared to FP+ (Soarin' and TT are being completely prebooked).