There are all sorts of reasons for arriving late. Some are voluntary (e.g. hungover and got a late start, had to stop at Starbucks first, couldn’t fit into desired clothes, etc) and some are involuntary (e.g. massive traffic jam, bad weather, sick family member or friend, etc). The voluntary ones don’t even deserve a justification: it’s pure laziness and to me that’s not a valid reason for being tardy. In such case, as I see it, the “obligation” is not met (and really would not have been met in pre-conciliar times either). Period.
But the involuntary ones are a different story. The person had the full intention of being on time for Mass but was impeded due to circumstances beyond his/her control. In such case, the law does (and did not, even in pre-conciliar times) not penalize, and I believe the “obligation” is met, even for a late arrival.
Merely my unsolicited :twocents: