It's a trend that is pretty much inevitable for a country where birth rates have been dropping starkly for decades, so that less people generating the taxable income that pays for those pensions enters the workforce, than those who leave it.

At the same time, medical advances make it so that retirees live longer than before: https://relojpublico.es/poblacion/esperanza-de-vida

If someone has solutions that would not cause widespread riots, I'm sure most goverments of countries in a similar situation would be all ears.

Spain will solve the problem by buying train tickets for all the pensioners.