Well
I disagree with your pretense that G.H. Bush Sr. was afraid to debate Bill Clinton, he certainly had the experience and knowledge to do that. Former CIA director, congressman, 8 years of being Vice President and President himself for 4 years. What's to be afraid of? I really don't remember him fudging on any debates with Bill Clinton. He did all 3.
What Bush Sr. had that Donald Trump doesn't have.
Experience & knowledge. He enjoyed having 16 other GOP candidates on the stage where he could launch insults at to divert attention. He attacked the media to divert attention. He backed out of a debate with Cruz & Kasich because he knew he could not run out the clock with just 2 other candidates and himself on the stage. That yes, he would be required to answer specific questions, and if he wouldn't be able to go into his typical platitude speak, (opening mouth words coming out, but never really saying anything) without the moderator coming right back on him to answer the question. Then if he answers it wrong--there would be two other candidates to make certain they should the nation how stupid he really is.
IOW Hillary Clinton is going to run him through a paper shredder. Trump is the 1st candidate in 40 years that has broken protocol and has now refused to release any income tax returns, so why not break another protocol.
And as we see, he may be trying to do that now.
Trump Is Already Trying To Find An Excuse To Skip The Debates
I mean really--this guy going toe to toe with Hillary Clinton--LOL