Thoughts do think they should have moved the achievement list over to tsw 5 because now any dlc you buy you cannot get the achievements if using tsw 5 you would have play tsw 4.
They decided not to. The reason they gave in the FAQ suggests they thought of the players who have been along for TSW2/3/4, but I don't quite buy that. Their main focus has always been new players, because a new player will likely buy older content that is TSW5 compatible, and achievements could have been an incentive for some players to do those revenue generating purchases. A more likely reason for the "clean slate" is that (a) it saved them some time and cost to not set up TSW5 achievements for old content, and (b) we can no longer complain that they don't unlock, when they don't unlock. By the way, when they slashed the number of achievements per route DLC to 3 at the launch of TSW3, they said it was because the number of achievements was too high. They also removed achievements from Loco DLCs. Yet, TSW3 had only 212 achievements (of which 2 could not be unlocked), from TSW2's 431 achievements (of which one could not be unlocked). Later, TSW4 had 260 achievements, so still quite a way off from the 431. But sure, much easier to prepare (copy/paste) just 39 achievements in TSW5, than to deal with 260+ achievements, some of which may be buggy and cause more headache down the road. But the total number of achievements has never been a blocker, as they claimed. This is proven by the sheer amount of achievements TSW2 had. No, the problem was more likely the cost and headaches.
