I'm of the opinion that this is a wonderful thing, for several reasons.
First, because many older pastors stay at their church too long. They build it up and then ride it down again b/c they don't have any exit plan, or any other way of supporting themselves. Almost every church I've ever seen pastored by a man in his late 60's/70's hollows out. OTOH, if he would leave before that hollowing out process the church will be in much better shape for his successor to then take to the next level. The key to that, though, is for the older pastor to have an exit plan i.e. teaching in a Bible college, an itinerant ministry such as you suggest, interim pastoring, a good retirement plan (HA! in the IFB world?), etc.
Second, this allows the older man to continue in ministry without the burden of pastoring. I realize many people don't view the pastorate as a burden, but if it is done right, it is. Paul referred to it as 'the care of all the churches', and though a pastor only has one church it is still a 'care.' But as those older men lose the vitality and forward leaning vision necessary for good pastoring that doesn't mean they have lost their usefulness to the cause of Christ of either. My father recently retired at 71 after 38 years in the pastorate and I'm working hard to do my part to see that he doesn't feel useless now...b/c he isn't at all.
Third, and this springs directly from the second, younger pastors and their churches can find great benefit from the itinerant ministry of an older man who has spent decades in successful ministry. Assuming his mental processes are still functioning he is at his peak of experience, Bible knowledge, and people knowledge. The people sitting in the pews stand to glean tremendous things from a man of such life experience and Bible knowledge. The younger pastor sitting there likewise has much to glean from the wisdom of that man. It is my considered opinion that one of the great mistakes my generation of pastors has made is to put the younger pastors of growing churches into the prime speaking positions at conferences. It damages those younger men, in many cases, and at the same time we totally miss out on the rich, rich experience and knowledge of the old pastors.
By all means, don't just let the old men preach, encourage them to do so.