How old are you 179club ?
I am 73 and at 71 kg consider myself pretty fit for my age. No cycling impediments except my age

I thought similar to you before I
purchased my Gain. Did I need it ? would it wreck my fitness level ? I had/still have a 8kg carbon road bike (which I still ride) I can still ride plenty of miles on my unassisted bike including some 12-15% hills providing they are reasonably short. What the Gain does is allow me the freedom to ride farther and higher without finding myself 20 miles from home and knackered. I have 'dialled' the lowest assistance level down to 80% and I almost exclusively ride at this level to help me maintain good fitness. Sure I have lost a little of the power and stamina I had even 18 months ago ! maybe riding the Gain has a little to do with that but at my age I accept it is inevitable. Incidentally the majority of my rides on the flatter courses would average 16 to 17 mph.
Here is my Garmin data from what I consider a flattish ride.
Distance 19.31 mi Time 1:09:05 Avg Speed 16.8 mph Elev Gain 663 ft.
I have cycled between 4,000 and 5,000 miles for the last 5 years and have every ride saved on Garmin Connect and therefore
I can plot my decline

as 'old father time goes to work on me.
A year is a long time (power and stamina wise) in one's 60's and 70's. When I look back at my rides from 1 > 2 years ago I wonder what happened to that man ?

but I will not stop until I cannot (for whatever reason) cycle. As time passes I appreciate my Gain more and more.
I still love cycling and ride between 80 & 100 miles every week. I am a great believer in 'use it or lose it'
Sorry, this post has almost turned autobiographical and I'm even beginning to bore myself.
Adieu
Roy