Rider weight massively outweighs the relevance of bike frame material, especially in the West where obesity epidemic has biased BMI upwards over the last half century.
Also, heavier riders are generally faster downhill as they have a greater terminal velocity.
No, unless they're Russian, they're not free falling. They have greater potential energy. And also increased traction, increased rolling resistance, and increased losses in wheel bearings and drive components due to friction.
Rolling resistance increases are pretty marginal with compensating tire pressures (i.e., higher). Wheel bearing losses are de minimis.
Drivetrain component friction is not a function of rider weight.
Wheel bearing friction, duh. If the rider stands up while pedaling, it does. :)
If you push 300W on a 5kg bike or 20kg bike, the "workout effect" will be the same.
At the end of the day, if you are looking for "workout effect", it means that you're trying to achieve something, like winning races or going faster.
And for a given rider, with a certain weight and certain physiological abilities, they will go much faster on a 5kg bike than on a 20kg bike.
There is a whole world between <$1k bikes and those >$2k ones. Weight being an important part. Not only because it reduce the total weight of bike + rider, but also because a 5kg bike behave in a very different way than a 20kg.
It's like saying that cooking with a chef knife is the same as cooking with a sword.
Sure, after a certain point, the race for removing a few grams here or there is a luxury and many people tend to believe that spending $$$ on a lighter bike will fix their lack of fitness.
But modern bikes provide a completely difference than the old, heavy ones.
Yes, rider weight trumps it, but modern bikes in general just ride nicer and most of us who are not pros only test a dozen different bikes at most. It's a hobby, people like to splurge.