Trump has already taken the step that effectively ends Obamacare. He has instructed IRS to accept tax returns even if they don’t include the mandate disclosure.
Without that threat of a substantial penalty, young people are not going to be paying these ridiculous prices for insurance they don’t want or need. Without them subsidizing the rest of the market, the system, as currently configured, will collapse in no time flat.
But as long as insurance companies have to immediately pick up pre-existing conditions when they write a policy, then "insurance" itself has been made illegal. That provision must not stand, but the GOPe squishes are no more interested in dealing with that than garden slugs are to go water skiing in the Great Salt Lake.