Yes, much of the GOP’s decline can be traced to Reagan, even if he’s been pretty much forgotten by rank-and-file Republicans these days.
He still had certain qualities Trump lacked: a genuine sense of humor, a willingness to work with Democrats, an ounce of contrition (he actually apologized on National TV for Iran-Contra, something Trump would never do), and a focus on a common enemy: the USSR.
No, Reagan wasn’t great, but he wasn’t an America-destroying menace.