A mathematical bent is a boon for many tasks, but making sense of ancient literature isn’t one of them.
I don’t see why a linear reduction would be more expected than something proportional. Why couldn’t biblical authors portray a decline by showing God taking a tithe of years away from each generation (something that would also plot a pretty curve)? I don’t think that’s what they did, but there’s no reason why they couldn’t.
When Jesus told three stories about lost items, he moved from many to few in the progression of 100 sheep, 10 coins, 2 sons. There’s no reason to have expected him to aim for a linear reduction by having 50 coins instead. In fact, probably all but accountants would see a clearer progression in 100-10-2 than in 100-50-2. Literarily and artistically, it just pops more.
When numbers span a wide range, what counts as a big drop will change depending on the starting point. A son who lives 100 years less than his dad may seem unremarkable if Dad lived to 969 but tragic if Dad lived to 119. A linear reduction would be an awkward choice for numbers spanning a range this large.
Math doesn’t get us far in seeing what’s going on with the ages. Literary considerations are at work. For instance, there’s another way of looking at how the big numbers in the Genesis 11 genealogy hover just above the century marks: except for the 119 at the end (which calls to mind Genesis 6:3), every 3-digit number contains a zero. The reason, I expect, is literary. In Hebrew as well as English, big numbers containing zeroes are shorter to spell out and say. This may also explain why Genesis 5 includes an extra phrase and number to total each person’s age, even though doing so doesn’t add any new information.
The patriarchs’ paragraphs in Genesis 5 look weighty (especially in a translation that writes out the numbers, reflecting the Hebrew). By contrast, in Genesis 11 they’re shorter to write and faster to say. That’s one more way Genesis conveys a shift from the giants of the past to people closer to us. It’s not a pattern to be reduced to an equation, but something to be experienced as an attentive reader or hearer.