Hypothetically, What would be the IMPACT of American politics if EVERY eligible voter ALWAYS voted?

The Dems would hold a lot more elected positions. Which is why the GOP is so keen on voter restrictions.

Speaking from a country that has compulsory voting, I think you should also think about preferential voting, just to shake up the duopoly.

Common wisdom is our elected representatives are in the most conservative 30% of the population and any survey of any social issue within and outside of parliament shows how out of step with current sentiment they are.

But with your balkanised electoral processes it's not difficult to see bad players getting around them (stacking the courts so that any voting dispute is judged favourably is a long term GOP strategy - Bannon has been rabbiting on about it for years).

You would see A LOT more programs for youth. Folks aged 18 - 24 are notoriously low percentage voters.

Turnout by age cohort from 1988 to 2016 shows people aged 60+ (heh, that's me) voted on average around 70% of the time. Folks aged 18 - 29 never cracked 50% and tended to hover in the mid-30s percent.

Per Wikipedia, in 2021, there were 43 million people in the US aged 15 - 24 (unfortunately, they don't seem to have a breakdown that eliminates the minors in this cohort), which works out to around 13% of the population.

Putting it all together (and ignoring the small year differences and lopping off 1/3 of the 15 - 24 group as being too young to vote), if the missing voters are added, we get:

18 - 24 about 15 M (43 M * .67 to get rid of 15 - 17 year olds, * .51 to account for missing voters)
25 - 34 about 20.25 M (45 M * .45 to account for missing voters)
35 - 44 about 17 M (43 M * .4 to account for missing voters)
45 - 54 about 16 M (41 M * .4 to account for missing voters)
55 - 64 about 13 M (43 M * .3 to account for missing voters)
65+ about 17 M (56 M * .3 to account for missing voters)

The cohort of ages 18 - 34 is a lot more than the 65+ cohort, about 80 M to 56 M. Youth programs and middle age programs don't happen because young people don't vote.

If you want to see student loans deferred or forgiven, get younger folks to vote. If you want to preserve abortion rights, get younger folks to vote. If you want expanded tax credits for people with minor children, better schools, or better long-term projects get funding (like eliminating fossil fuel dependency), then get younger folks to vote.

If you want more money to go to cancer care and Alzheimer's research, then embrace the status quo.