They want to win the 2030 WC bid, and Messi has already signed as an ambassador like you have alluded to.
He's in the last years of his career so why not take the money? He's won everything at domestic level.
Criticise Ronnie all you want, I haven't liked the guy well before he left United in 2009. Don't pretend like you wouldn't accept that contract either. Nobody would decline a £170m a year contract.
Also, many people are quick to hate on the gulf region, and gulf states in general but have never visited. We form our opinion purely based on the media. I've visited its very nice, very hospitable, Saudi obviously has a very patchy record.
Reality for Saudi, for Qatar for any oil rich states is the following... it's not how long the oil reserves last, it's how long the world actually needs them. It's nonsense to have ~220 years of oil reserves when the world probably doesn't need as much oil in the next 30 years, maybe for plastics and a few other things?
Fusion was achieved a few weeks ago for the first time in a very small scale, billions are poured into private companies now and we'll probably be there in 10-20 years, with commercial plants that follow.
The world won't be reliant on fossil fuels as it is today, logistically we will have things like electric / autonomous cars where we don't own cars but its subscription based where we get picked up and taken places.
We won't be reliant on that, so gulf states need to expand fast use their money into things like sports, real estate, education and build other industries/economies which Saudi is doing now with tourism, with buying football clubs for instance.
They have to invest in the right assets, many of these countries had nothing before oil so they have a golden opportunity to get the best infrastructure in a very small time window. We're talking one generation here, so they have to import labour, skills, everything.
These guys know they need to invest their money in the right places and using such a global name like Ronaldo is worth £170m a year. Even if his legs have gone, I think Saudi might even try to compete with MLS in the long run.
We also live in countries that colonised the world historically so we're in no real position to judge, and the products we buy are invested by the same investment groups that politically we'd be completely opposed to.
I think 99% of people would take that £170m a year, I'd be on a flight to Riyadh if I got offered that kind of money.