That isn't the reason. They sunset in the bill so it has a lower CBO score (which calculates costs out to 10 years). If you sunset in the bill after 5 years, even if you know it will get renewed, the apparent cost goes down. Get it?
If anything it has been the opposite problem, with modern congresses having been more than happy to delegate away their powers. You might have heard the recent tariff news for example.
Modern being what? The main tariff laws in question happened 50+ years ago. The republicans in congress have been going out of their way to not interfere, but that's significantly different from creating new delegations.
Since each person has one vote why would outliers matters? It doesn't seem that length of terms doesn't even help with leadership roles The Democrats minority leader is Hakeem Sekou Jeffries who was elected about 12 years ago and Mike Johnson has been in the house for 8~ years. There are R and D members who have been in the house longer.
