And those same republicans are in support of alternate methods to ensure pre-existing condition protection. That is the part you are ignoring.
And those same Republicans have no idea how healthcare works, evidently.
The ACA was an improvement, albeit far from ideal, and the US still arguably has the single worst healthcare system of any similarly wealthy nation, no offense.
The US' healthcare spending per capita is vastly higher than any other nation. Both the coverage and the affordability are strongly inferior to other wealthy nations' healthcare systems (Canada, all of Scandinavia, UK, Belgium, Germany, France, etc. etc.) and all of those examples provide universal affordable coverage. Don't get me wrong, the quality of care in the US is by all means excellent, but it's also extortionately expensive compared to any other country.
It's not like other countries all use the same system either. Ours is much better and we have a multipayer universal healthcare system, not a single payer one. A true single payer system is very rare for the record. Our public healthcare mandate ensures everyone has some degree of basic universal coverage and private insurance can supplement it to whatever suits your needs if necessary. I have 100% coverage for almost everything due to my supplementary private insurance.
In order for a healthcare system to work, as all those countries have figured out long ago, your healthcare pool needs to be as large as possible and include as many healthy people as possible in order to make the system work. A mandate is of course the easiest way to maximize the pool size.