Disastrous error in judgement to be certain. I'm not sure why in the rough & tumble world of the 19th century, where life expectancies for even the wealthy and powerful were shortened (by today's standards) and difficult to predict, why Alexander would have run the risk of a younger unprepared son and successor.
I also wonder however how much it would have mattered though. Nicholas' ultra-conservative ideology seems to have every bit as much to do with his blunders and downfall as his general unpreparedness in assuming the throne. Clearly he didn't get the most out of his "on the job training" but you wonder how, going on twenty years leading up to the start of the World War I, he could have had continued to be so poorly suited to his role of leader, legislator, and commander in chief...I sometimes get to thinking that he possessed a crippled mindset that couldn't have been altered or improved upon in a century, much less twenty years. Although by the same token he was also intellectually submissive...so who knows I guess :-/