Patents can be useful as an incentive, but their length need to correspond with the pace of progress in a given field. Otherwise, they end up slowing down everyone.
Once you have proven the need for an exclusivity grant, You also need to prove that 5, 10, 15 years of exclusivity via a patent would be unable provide enough incentive, that it had to be 20 years to strike the proper balance
In today's fast moving society, I think 5 years would provide more than enough incentive even for expensive technology that requires lots of capital investment though I would favor a shorter exclusivity grant, and then may be longer required licensing grant. like 2-3 years of exclusive use, and then 10 years of licensing but the patent holder is required to treat all uses of the patent in the same manner with fair and non-discriminatory pricing.
Now that is not to say I would advocate for complete removal of the patent systems but I do think Compulsory FRAND style licensing should be a requirement of obtaining a patent
Your distinction between Copyright and Patent is also strang as the suffer from the same flaw so it seems your only justification is that you believe 20 years is "reasonable" but Life is not
I think both are unreasonable, I would personally like to see both dropped to 10 years, or some compromise where you get 2-3 years exclusive use of the creation then have some kind of compulsory license where the creator is compensated but can not with hold the creation for 20 years (with some kind of scheme that the license be fair and equitable)