We all knew TRUMP wouldn't testify--COULDN'T testify! Every lawyer among his own AND in pundit world knew he would PERJURE himself and trip over his own tongue and then LIE some more, LIKELY putting himself in MORE legal jeopardy.
So you are correct, this was the ONLY course of action--let someone go and try to finesse Mueller to get TRUMP out of having to appear or be subpoeaned. BECAUSE TRUMP would lie small and BIG. HOWEVER, McGahn can't make all the Russian contacts, Russian monies, international bank transactions, staggering indebted Republican campaign chairmen doing side deals to bail themselves out by getting changes to the Republican NATIONAL PARTY PLATFORM, drunken pimply faced campaign braggarts telling English diplomats in bars that his presidential candidate was getting help from Russia to win, or kids knowingly setting up meetings with Russians to get political dirt on their dadddy's opponent.
Do you REALLY think McGahn could be that GOOD? And don't blow smoke up our keyboards. None of us on this board know what Trump told McGahn to say or NOT SAY. No one knows what McGahn will or won't say!! White House Counsel straddles 2 clients--the president and the office. Depending on HOW he wants to or can step over the line, HE KNOWS TRUMP LIES, about everything, not the least of which why Trump FIRES PEOPLE. So how far will this guy go for his "client"?