> Is it safe to say that people are generally much worse at mental math nowadays due to calculators being ready available on every smartphone? I think it is reasonable to assume that's true. Math is more accessible thanks to tools now, but people are worse at it in general?
Nope. The answer is government and money.
As someone who had a father that taught students and now has kids (that I teach myself, and help them go to school) I know the direct answer: teachers. Also: mental math did not drop off a cliff when calculators appeared, nor when they were accepted. It slowly slid downward. More than that: it peaked. It's not like the people who fought in WW2 were good at mental math either. So between 1960 and 2000 something happened to greatly push mental math ... and now to destroy that ability.
A long-term perspective on the teacher profession also taught me why. In the 70s few people were educated. Why would you? And worse: we were all working in industry. That's where the money was ... but not if you were educated (which, let's be honest, tends to instill less willingness to do repetitive manual labor). The limiting factor of growing industry was: geometry, calculus and fast mental math. That's what you require to make good plans and good use of factories to turn metal into products.
A lot of smart, or at least educated, people were lured into government positions in the 70s by the previous economic crisis, and the new growth market of industry were pushing and pushing and pushing to teach math, from as young as possible. They taught themselves math because they started teaching careers and the money in these was hard to beat. Industry paid for advanced math courses for teachers. In the 70s teaching positions easily matched industry wages, except you mostly worked at a desk, and especially the pension plans were sweet. Teachers got free math course, paid for by industry, to teach themselves math because that got you better positions, making a bit more per month.
Of course, the government lied. Here's the HUGE caveat in a government contract: A government can simply vote in a law or even regulation that "overrides the contract", an therefore cannot be counted on to keep long-term promises even when there is a signature on a paper. And while unions help slow the destruction, they cannot stop the slide. So starting after the crisis at the end of the 1980s governments lowered teacher pay, even for existing people (which is where government power comes in: private sector firms are not allowed to do this). Which then also meant they accepted a much lower quality of people into the teaching profession, as they couldn't compete anymore, the government simply did not care, and existing teachers got discouraged. Now, mind you, "quality" refers to their academic ability, as certainly a part of teacher pay was (and is) that you work all day in a position where you socially dominate everyone around you, which of course led to abuses of that position as well. And since it's not like lower academic quality people are not abusing their positions as teachers, that part has stayed (just look up developmental psychology papers: almost all child abuse happens in schools, and what has the government done about that? Well: CPS was legally forbidden from investigating schools, or teachers. Only parents get investigated and even blamed for school problems (that's in theory, in practice of course it's mostly the least powerful that CPS really attacks: the kids themselves))