The old four group system was awful, and I was so happy when they changed it. It was even terrible as teams lost their result against the team that did not advance.
I also like 16 teams, as it allows the mid-tier teams like Switzerland, Germany, Latvia, Slovakia, and I guess you can include Norway and Denmark to consistently play top teams. If the WC was not 16 teams where would some of these teams be? Germany has shown great progress recently, and if they make the QF this year, that will be 3/4 years they have. Slovakia used to be way ahead of those other teams, but the last fifteen years has allowed them to catch up.
The issue at the bottom will never fully go away, but has been exacerbated by the decline of Belarus primarily. Belarus used to be in that mid-tier, but have fully fallen off that wagon lately. This has forced teams like France and Austria, who normally would be in a relagation fight up. The other change that had helped teams like Great Britain and South Korea come up is the switch in how Division 1 works. It used to be two divisions with one promoted from each. This typically meant a higher gap in talent from top to bottom. The teams that were recently relegated, typically had large talent gaps on at least 2-3 teams in the 6 team groups, and could, regardless of unity/cohesion, win those games easily. It would usually come down to a one game showdown with another team, which was usually scheduled last. Now with it being the next best six in one group, and two being promoted from there, it is very close and there is little room for error. So teams like South Korea, who, especially prior to the olympics, had a significant amount of time to gel, were able to take advantage.
I think there is merit in having the promoted teams play the relegated teams, but at the senior men's level this is less of a concern. At the u-20 and u-18 levels, the promoted teams and relegated teams for the next year, should playoff, as roster turnover is significantly higher.