It doesn't actually need to become the best league, it's an extension of Saudi soft power. So long as the league and some teams become prominent it is working. How many Saudi teams could most people name before this year? I could only name one.
I'd also argue that many of us are not the primary target audience when they are doing this. I'm not sure if it has been noted by many outlets but a lot of the players the Saudis are going for are Muslim: Benzema, Kante, Mane, Mendy, Koulibaly, Mahrez, Mousa Dembele, Fofana from Lens; they have been linked to or attempted to sign more such as Pogba, Ziyech, Partey, Dzeko, Gundogan. There is probably more out there but those are the ones I can recall. Ronaldo is also incredibly popular in the Islamic world, moreso than Messi for some reason.
The Saudis like to position themselves as the leader of the Islamic/Arab world, and this part of that strategy. They want the biggest league in the Islamic world, where the biggest Muslim and Arab players are.
There won't be many people from the UK who begin supporting a Saudi team, but there is a massive market of people in Africa and Asia there to capture (one which is not as averse to supporting multiple teams as your typical European supporter, worth adding!).
Building up a domestic sporting league helps diversify their economy away from the reliance on oil, brings in money from overseas and draws in tourism, and really increases the profile of their country in parts of the world which they are very interested in extending their soft power to. The Premier League has given England a massive boost in soft power for decades now, even a fraction of that is going to help the Saudis a large amount because they are working from a much lower base.