My understanding is that the SOP for low fuel is that you need to declare a fuel emergency (i.e., "Mayday Mayday Mayday Fuel") one you reach the point where you will land with only reserve fuel left. The point OP was making is that the entire system of fuel planning is designed so that you should never reach the Mayday stage as a result of something you can expect to happen eventually (such as really bad weather). If you land with reserve fuel, it is normally investigated like any other emergency.
Flight plans require you to look at the weather reports of your destination before you take off and pick at least one or two alternates that will let you divert if the weather is marginal. The fuel you load includes several redundancies to deal with different unexpected conditions[1] as well as the need to divert if you cannot land.
There have been a few historical cases of planes running out of fuel (and quite a few cases of planes landing with only reserve fuel), and usually the root cause was a pilot not making the decision to go to an alternate airport soon enough or not declaring an emergency immediately -- even with very dynamic weather conditions you should have enough fuel for a go-around, holding, and going to an alternate.
[1]: https://www.casa.gov.au/guidelines-aircraft-fuel-requirement...
Landing at an alternate location is significantly more expensive, so I assume Ryanair put pressure on its pilots to avoid that…?
We'll find out in the investigation, but "get-there-itis" is a very common condition amongst pilots and can lead to them delaying making decisions (such as going to alternates) so it's possible that this happened without explicit (or implicit) pressure from management.
That being said, the fact that (AFAICS) they first tried to divert to a closer airport where the weather was similar rather than an alternate with clear weather was probably one of the causes of this event.