If Nixon hadn’t resigned, there would have been zero chance of a President Reagan. The GOP nominee in ‘76 would have been Nixon’s successor - perhaps John Connally or someone. And they probably would have won.
Watergate or no Watergate, Reagan was planning on running for the GOP nomination in 1976. He had made a stab at the GOP nomination in 1968 in an effort to stop Nixon. By default, Jimmah Carter turned out to be the anti-Nixon/anti-Watergate/anti-Vietnam War candidate. Had Nixon survived Watergate, the GOP would've been in decent shape for a serious effort by Reagan to become the nominee. Maybe with Connerly as his running mate.