Yes, of course, after the team sold most of the high earners after Koeman's first season to reduce wages (Messi, Griezmann and so on), Xavi got some good money to invest by La Liga standards (not by Europe top teams standard, that's a different playing ground). That's surely a big part of the reason why the team performed better domestically than when he was named.
But... the team achieved important things with Xavi in the league. It is a simple fact. Took over the team in a bad time and got it to domestic success. It doesn't get much better than winning a league title vs a Real Madrid team that in the previous season achieved a La Liga + CL double. This is simply recognizing a simple fact and not letting bias to affect your thinking.
Now, if a new manager comes in instead of Xavi, his job will be to improve the team on short term, and very soon to win titles. Exactly like Xavi did. Even matching that is not a walk in the park as some think. It takes some skill to actually win a La Liga title against a top 3 team in Europe like Real Madrid.