As you said, the fact is that Al Hilal does have an extraordinary squad depth. They have performed consistently in recent seasons, which is clear evidence that their presence in the Saudi League can be a real threat to other teams, including Al Nassr and Al Ittihad. I think this is where the excitement lies, each club is striving to continuously strengthen its squad, whether by signing European players or by improving the performance of each player to ensure they perform at their best. The presence of so many star players in the SPL should motivate other players to compete healthily, proving that they also possess quality that is no less than that of European players.
For the strength and level of their league, they do have a fair squad, AlnHilal happens to be one of the top five teams so they are doing well domestically even if it is not beyond Saudi Arabia. One of the reasons you see then going for top European players is to enable them improve the quality of their league because they know with the players they gwt from their domestic leagues, they can hardly get the same kind of quality they will get with the European players.
That is why the Saudi entry clubs have invested the most in European players, keeping foreign players in order to improve their local players and gain all kinds of experience. Among the players that are in their own country's domestic clubs, there is a small number of European players, players from their own country and European players are definitely needed to improve the reputation of their country and the quality of the club. For example,
Al Nassr club is best known for accepting Ronaldo, although they have spent a lot of money but are making local players widely experienced.