You're correct that new growth cedar is not as durable as old growth. However, in my experience, the main problem is with cedar shakes (roofing), which is an application where they are more subjected to accumulated debris which retains moisture and promotes rotting. Since shingles are applied in a vertical position, and have no problem shedding water if they are installed properly, you should get a decent lifespan out of them...maybe not as long as 50 years ago, but still adequate.

I have unpainted white ceder siding as do lots of houses on coastal MA and they aren't being replaced every 15 years.
While new shingles may not be as good as years ago I think the main issue is the exposure to weather of the install.
With the old shingles you may have been ok with 6 or 7 inches exposed I think 5 inches now is better.
I don't expect to replace my siding in the next 25 years and I don't think you need to worry either.

Besides, if your current shingles are rotten, they're not going to last 25 years either.

they have synthetic shingles that look like cedar shakes and come with 50 yur warranty.. probably expensive though..
look up davinci roofscapes for one example

New wood..new growth is not as good as old wood.