I can’t judge the state of Portugese public transport (apart from the fact that the lissabon-madrid rail link is a tragedy, mostly thanks to portugal), but flying is very obviously a big-city thing. Inhabitants of big cities fly much more because the airport with frequent and direct flights is right around the corner as opposed to rural places where the nearest airport is quite some distance away and serves few destinations.
In general poor people fly very little, which is also the case in Germany, so I can’t imagine it making much of a dent around there for the poorest. Portugal is itself mostly responsible for its transportation network and, if I’m understanding https://www.railjournal.com/in_depth/road-out-rail-in-under-new-portugese-plan/ correctly, the portugese government has chosen to invest in its road network over rail in the recent past. While just looking at the cp website it seems that prices are pretty low compared to germany or france. Similarly for hostels it seems porto and lissabon are cheaper than many less touristy cites like lyon, toulouse, cologne, genoa, … right now. I just can’t imagine it being cheaper to fly outside of portugal for vacations based on those prices. And at least for gas taxes there certainly is an alternative without large changes that is especially viable for non city-dwellers: electric cars. While still too expensive, they are much cheaper than even 5 years ago.
The last point is entirely ridiculous: The Netherlands certainly isn’t known for cheap trains and france is the opposite of a train every 10 minutes (especially outside paris), with often large multi-hour gaps between TGV connections from many cities. Most people in other european countries fly much less than people in Portugal or Spain: Portugal and Spain have one of the highest per-capita flight rates in europe (https://ourworldindata.org/grapher/air-trips-per-capita), more than double germany or france, even though spain has one of the largest HSR networks (in total and per-capita) in europe.
Flying is one of the few climate related things where the only foreseeable “solution” is a reduction. For heating, electricity, driving, steel, … there are technological solutions in the works and often already deployed that should solve the problem in the next 20-30 years. For flying there is little in the works. The aviation industry talks about SAF while missing targets for implementing them or even thoughts on how to deploy them on a large scale, while mostly ignoring the various non-CO2 related effects flying has on the climate. Flying related emissions mostly increase year-over-year (due to increasing demand) without indications of reduced emissions in the near future. And with flying mostly being for leisure, it is doesn’t need to be directly replaced.
And just to repeat myself: Flying just mostly isn’t a thing poor people do.
Cross Border Rail in the EU is surprisingly bad, IMHO. If you stay within one country, prices are lower, frequency and capacity are higher and usually the speed is higher as well. Eurostar has decreased in capacity over time and is by far the most expensive option, Thalys hasn’t increased capacity in decades (bought no new trains) and is also very expensive, ICE services to Netherlands and Belgium aren’t cheap and suffer from bad rolling stock (often half of the trains are cancelled), all border connections to spain suck in some form (Madrid-Lissabon is especially bad), there are (almost) no trains through switzerland (to connect to italy) and trains in switzerland are in general pretty slow, making trains to Italy very unattractive. Connections to Denmark suffer from bad rolling stock and are often booked out, there is no way to cross the Baltikum in any reasonable amount of time by train, there are very few train connections left in the balkans after Covid (e.g. greece has no single passenger train crossing the border). For people trying to cross over from Sweden to Finnland over Land, there are trains running to the border towns, but a walk of several kms is required between them (or there may be some form of bus connections, but I can’t figure them out). And everything train-wise is very slow in eastern europe anyways. Night trains are very often sold out far in advance and have historic rolling stock with lots of issues.
Worse than that, international tickets are often much more expensive than national ones (compare e.g. Frankfurt-Paris to Munich-Berlin), the border crossing is often the slowest part of the journey and if you want to get the cheap prices on both sides you have to book separate tickets, with all the ensuing fun of figuring out what to do if something goes wrong. Sometimes you just can’t book a single ticket anyway. There isn’t a great europe wide journey planner (good luck with Lisbon-Madrid or the Euskotren border crossing) with real-time information.
Cross border trains often aren’t the priority of a single country (for obvious reasons) and the entity that is supposed to have an eye on eu border travel by rail (the EU) frankly just doesn’t care a lot and it shows. The passport controls at internal shengen borders leading to 30+ min delays are just the cherry on top.
Still: A lot of intra-EU flights can be reasonably done by train/long-distance bus and a lot of (especially holiday travel) destinations substituted. It could always be much worse.