In your mind, how does dropping leaves harm the shrubs? Wouldn't it allow for more light to pass down to the forest floor? It feels like the tree's response to shrubs was "grow taller to maximize sun exposure"
I feel like the conventional wisdom that this is a response to environment(conserve energy/lower wind resistance) make more sense than "offensive deterrent for competing plants" even more so when you consider that every leaf dropped is biomass that the tree had already taken in and utilized, but now it's giving it up. Maybe a pro-symbiotic move whereby dropping the leaves provides shelter/biomass for beneficial insects that could provide root aeration or soil enrichment would be another reason?
I think conserving energy in cold months is close to the biggest factor.
Regarding the dropping of biomass, in both deciduous and evergreen species, some of the substances from the retiring leaves/needles are reclaimed by trees ("retranslocation") and pulled out of the leaf before letting go of the leaf. In species like pines it'll be the second or third (or older, sometimes much older) retiring needles and in species like maple or beech it'll the be the entire foliage. Mass useful to soil ecosystems still falls but the tree grabs what it can in the late months of the year, hence the color change.
One thing the grandparent comment doesn't talk about much is the cost (in sugar) of both building and thereafter maintaining leaves and the related tradeoffs. Building a fully winter-tolerant broadleaf is more expensive sugar-wise than building a winter-interolerant leaf. A sugar maple in Quebec, where the season is shorter than (say) Oregon is going to compete in its niche better if it can attain surface area quickly at the start of the growing season, and that is better done with a winter-intolerant, more relatively delicate leaf. An evergreen leaf takes more time and mass to build and has to have more limited surface area or armor (cuticle) to tolerate such winters.
Decaying leaves do change the ph of the soil and can leave a wet heavy mass that new sprouts have trouble sprouting through and seeds have trouble getting through to the soil.
There is no reason shedding leaves has to only be for one single purpose like weather protection, saving energy, or trying to prevent competition, it can be all at once.
I'm largely going by what I see in my yard. If I want any of the ground shrubs to not die every year, I have to make sure they are not drowned in leaves.
Note that I'm not claiming the leaves are necessarily bad. They make pretty good mulch and such. But, without tending to uncover the smaller plants, they do a pretty good job of killing the things below the tree.
It affects seedling recruitment for sure. Some species love to start beneath leaf mulch and there is probably something out there that would colonize your leaf field hard and fast if it were given the chance. I collect red alder seedlings in timber clearcutting areas, and often find that they've had to twist/push their way through 6-12" of slash (clearcutting debris) to reach light and finally grow upwards. Quite a few conifers can push upward through many inches of needle duff as well.
It occurs to me that blackberries have entered the discussion. :D They seem to be the prime thing that doesn't give a crap about leaf cover around here.