La liga as a whole is going to struggle compared to past with or without Messi which is inevitable soon.
The quality is as low as have seen it across the board and the lack of pace/power is dating it a bit.
Barca/Real are not going to have the huge pull as in past and EPL will be very attractive destination with cash across league to back it up.
Next years going to be difficult for la liga teams and can see star played preferring EPL more and more.
It goes in cycles. Serie A had a golden age in the 80s and especially the 90s into the early 2000s. A 20 year peak and then it came crashing to a halt.
La Liga's golden age was the 2000s to 2020. Real Madrid with 5 European Cups and Barcelona 4. Atletico and Valencia reaching 2 CL finals each and Villareal and Deportivo a semi final (most of these clubs have had a sharp decline on and off the pitch). Sevilla with 6 Europa League's and Atletico 3 and Valencia 1. The Real Madrid Galactico side of early 2000s, 3 CL titles in a row. Barca with arguably the greatest club side ever under Guardiola and at least one of them. Messi and Ronaldo. Ronaldinho and Zidane.
Messi's departure will confirm the end of that, although in reality Barca's decline has been years in the making and the champions Real Madrid have been schooled in the last 16 by Ajax and Man City in their own ground in the last two seasons.
Further to the Serie A comparison is the decline of AC Milan which Barca are determined to repeat. While Barca and Real will regroup and be better again eventually, the rest of the league is really struggling apart from Simeone still going at Atletico and Sevilla's Europa League love affair. The fact neither could get near a mediocre Real and a poor Barca all season speaks volumes though.