Nor did we have the data to say it WAS necessary, but we sure as hell could predict most of the negative social and economic side effects easily. Causing profound developmental delays in an entire generation of children and lowering lifetime educational attainment for almost every single teenager was worth it though, I guess.
We had data from December 2019 until April 2020 showing that it spread quickly and can kill you.
We also had data that strong action can stop an outbreak (i.e. Aviation flu, Swine Flu, Ebola, etc). Sure when we took those actions early enough to curtail the outbreak it leads to people thinking the actions were useless (see Y2K debate) but it doesn't actually mean the actions were useless. People only notice when the dam collapses, not when it doesn't.
By March 2020 we had the Diamond Princess Data which established an upper bound of 1% case fatality rate (CFR) for COVID. We already knew then that it's definitely not the Spanish Flu (2.5 - 10% CFR), not Cholera (3%), not Smallpox (3%), not SARS (11%), not even Measles (1%-3%).
Not only did we never do anything like COVID measures for any of the diseases you mentioned but the WHO described the individual NPIs that would later make up the COVID "lockdowns" as "never recommended under any circumstances" in their 2019 flu pandemic recommendations.
What happened with COVID was an unprecedented overreaction by historical standards where we threw overboard everything we knew to try authoritarian gobbledygook on the back of the carefully cultivated FUD around COVID coming out of China.
The absolutely devastating economic and social effects were known, the possible effects of the disease both short and long-term were unknown. So we decided to go for the known devastating effects in case the disease was also devastating? That makes zero sense. And there were alternatives proposed to lockdowns like the Great Barrington Declaration that were not simply not considered, but were actively suppressed and smeared by government officials and their media lackeys.
“This proposal from the three fringe epidemiologists . . . seems to be getting a lot of attention – and even a co-signature from Nobel Prize winner Mike Leavitt at Stanford. There needs to be a quick and devastating published take down of its premises, is it underway?”
-NIH director Francis Collins in an email to Anthony Fauci about the Great Barrington Declaration
Great Barrington -> 4th of October. Are you even reading my comments? I am referring to the first lockdowns which took place in April 2020.
"So we decided to go for the known devastating effects in case the disease was also devastating?"
Of course we do in case of a pandemic, since we are dealing with a completely different underlying distribution of effects.
In itself your statement is accurate: we did not have data that is impossible to have.
In broader context the implication is not correct. We had experts consider long and hard what to do in the case of an influenza pandemic and the consensus for decades had been to disrupt society as little as possible.