I suspect you are right. Mueller will follow the evidence wherever it leads. If he feels he needs the tax returns he will get them. 

I am sure Trump is concerned there are other issues in the returns other than the Russian issue. Clinton was not impeached over Whitewater but about lying under oath about Ms Lewinsky. Nixon was almost impeached, not for the break in at Watergate but rather the cover up.

You never know what a special prosecutor will find when he starts digging into your life.