I don't know the official answer to the OP's question of what the point is that passengers are no longer required to buy before boarding, but what'd seem fair is to treat excessive queues as a delay. Many (all?) TOCs publish waiting time targets , such as:
East Coast: Our target is that you need not wait to buy a ticket longer than five minutes during peak times or three minutes at other times. Peak times are shown on station information posters.
Scotrail: You will be served at our ticket offices within three minutes at off-peak times. At peak times it should take no longer than five minutes to be served. We will publish the times of these busy periods at each station.
If these were treated as a delay for the purposes of Delay-Repay/etc, then it wouldn't remove the need to keep queuing, just like other types of delays don't allow you to travel without a ticket - but it may mean its in the TOC's interests to announce "the next service from platform XXX will have pay-on-train allowed today, due to large queues at the ticket office" instead of paying out the DR
I don't believe this could work with the current targets, as they seem very optimistic in some cases. Perhaps TOCs could have a "should/aim to" time and a "must/will" time.