He was desperate to manage abroad, it was an ambition of his- he probably felt it was ideal if he left on a high. I've never blamed Heysel for our situation, ever, personally.
Perhaps he was. He certainly said as much in his autobiography. But let's put it this way. As he also said in his book, Barcelona first contacted Liverpool about taking Kenny Dalglish as manager. Their Chairman of the time politely declined the request and pointed Barcelona across the park to Everton and their fine manager Mr Kendall...
Once more, this illustrates why they maintained their position at the top of the English game for another 5 years - and then returned again in the last decade - while we withered away and, effectively died as a force.
Howard might well have gone regardless. Maybe his ambition got the better of him (he never really made a huge success of managing abroad), but I don't think the club really did enough to keep him - or replace him if they couldn't keep him. Asking Colin Harvey to step up was, with the best will in the world, convenient and a little lazy.