I like the concept of qualification through rankings, if only to remove all the frustrations about criticism of team selections, and to provide a clean and transparent process. It gives those people who believe they will never get picked a chance to earn their place.
However, as said above, this only works if the team event does not clash with the sides. As also said above, as a system, it would be totally unfair to the England player pool, many of whom might significantly out-perform Welsh, Irish, or Scottish players over the course of the season, but due to the sheer volume, they wouldn"t get a place.
Something not mentioned above is the "exclusivity" within the rankings system in terms of opportunity. Some of the ranking events are not open to all. OK, hypothetically they are open, however some people cannot fund trips to Prague, Vegas (or even Ireland) to play those ranking events, hence those with deepest pockets would have an advantage at the rankings and therefore a headstart toward country selection.
Lastly, by setting the rankings as a selection policy, we would create an un-level playing field within the competition, ie not all teams have 4/5 players in the rankings, and therefore have free selection opportunity. Not sure if this makes a difference ultimately when the competition is played, however it is disparity within the process, and potentially an advantage to one or the other.