I admire your enthusiasm.
You might care to temper it with a balance of rigour in verifying your own beliefs and anecdotal experiences, your haste to dismiss that of others, and as in assessing your own methodology and its potential weaknesses.
I could perhaps have been more clear to indicate that use of acidic, pulp-based paper is more common in paperback publications, rather than universal. The point remains that as a cheaper publication mode, that cheaper publication processes and materials are more prevalent. I have encountered issues with pulp-based decay in both paperback and hardcopy books.
I'd linked an 1898 reference to issues with high-pulp, acidic paper degradation in an earlier comment (also submitted as an HN item). It begins:
The Library of Congress is indebted to the American ambassador at Berlin, the Hon Andrew D. White, for the following copy of the regulations adopted by the Prussian Government for the security of the national archives, and teh special danger involved in printing or writing records on paper made of wood pulp.
Wood pulp is extensively used in the manufacture of modern paper.
Paper made from compositions containing wood pulp decays more or less rapidly in proportion to the amount of wood pulp used.
Such paper is unfit for official use where permanency of records is essential or important.
<https://babel.hathitrust.org/cgi/pt?id=mdp.39015036735036&vi...>
The specific issue is somewhat less the wood pulp than acidic materials used in its preparation and their interaction with the pulp. Wikipedia has a good article on the matter:
Paper degradation is a slow process, but it is significantly accelerated in an acidic environment. In the mid-nineteenth century, the method of paper production became popular, in which resin-alum glue was added to the paper pulp.[3] The aluminum sulphate remaining in the paper form, in reaction with water, acids that catalyze the decomposition of cellulose (acidic hydrolysis). In this process, the cellulose chains are shortened, which reduces the tear resistance of the paper, and at the same time increases the cross-linking of their structure that causes the paper to stiffen and become brittle.[4] Parallel to the degradation under the influence of water, the cellulose chains react with oxygen, in result of oxidation the chains are also shortened.[5] Not only cellulose, but also the lignin contained in the paper is oxidized, which leads to the yellowing of the paper.
... The process of self-degradation of paper causes exceptional difficulties in safeguarding the collections of archives and libraries. For example, an analysis of the book collections of the Jagiellonian Library, Adam Mickiewicz University in Poznań, Książnica Cieszyńska, the AGH University of Science and Technology and the Cracow University of Technology proved that as much as 90% of the resources published by the mid-1990s (to be precise in 1996 in Poland) have all the features of acidic paper.[7] It turned out that these institutions, established to care for the heritage of the past, are not able to effectively carry out their mission.[8]
<https://en.wikipedia.org/wiki/Acidic_paper>
As to books published in the 1970s and 1980s, I have, or have had, numerous instances of these in my own personal library, purchased new, which are in the process of or have entirely degraded beyond usability, the latter having been discarded.
Those Spanish examples you've encountered might be a useful foil on which to consider a further concept (and common methodological / sampling error): survivorship bias:
<https://en.wikipedia.org/wiki/Survivorship_bias>