I think the argument is that a product that is hard to repair is less likely to be repaired, and, thus, more likely to become waste and be more quickly replaced

I think the argument is that a product that is hard to repair is less likely to be repaired, and, thus, more likely to become waste and be more quickly replaced with an altogether new product when confronted with a minor, and otherwise repairable, fault. For an individual user who may be disinclined to repair a machine, it may make no material difference. For society as a whole, it has an impact. Of course, if the overwhelming behavior is disposal, repairability may have negative environmental impacts (extra material use for modularity, etc). A similar example would be people discarding reusable plastic bags after one or two uses, resulting in more plastic waste than with bags intended for single use. It takes time for markets and users to adjust, and we may find that Apple was well-positioned to take advantage of market trends in processor and efficiency improvements at the right time for portable device markets like phones and laptops. Maybe we'll swing back to longer lifetimes and more modularity at some point...