People express all sorts of strange objections to bringing an end to the disability, pain, suffering, and death caused by aging. I think one possible reason for these objections is that they are a form of displacement activity, a way to put off engaging with the uncomfortable topic of declining health and death. So there are complaints about unimportant things such as the possibility of boredom, possible changes in the arrangement of society, and whether or not an individual would keep the same job for a century - whether our society would become absurdly static. As these silly little debates take place, more than 100,000 people die of aging each and every day. Were that the result of natural disaster or plague, you can be certain that it would dominate the headlines, and there would be an outpouring of support for efforts to address the issue. Few voices would be suggesting a halt to this work because it might alter international trade, or because the focus of some institutions changed as a result. We all know where the importance lies here: it is a matter of life and death. So too with aging.
Rejuvenation biotechnology would allow older adults to continue working and producing wealth for much longer than they can today, thus benefiting society in many ways. However, some people are concerned that this might do more harm than good; imagine all those rejuvenated elders holding onto their jobs forever, preventing the young from getting jobs themselves! Not to mention the risk of a gerontocratic world, where powerful older people get a touch too attached to their chairs, never allowing younger people a chance!
Quite frankly, what's wrong with that? Just because someone has been in charge of the same position for long, it doesn't mean that it's necessarily a bad thing. If you think otherwise, you might be making the incorrect assumption that, rejuvenated or not, older people will always tend to do things in old ways, eventually making them a worse choice than younger people. On the contrary, their long experience might make them more fit than others, especially if we're talking about chronologically older but open-minded people who keep up to date. Personally, I think what matters is that people in certain positions, whether within government or a company, are the right people for the job. If they aren't, old or young, they should be replaced by other people who are more fit, and, generally, there are more efficient and humane ways to do so than letting them get age-related diseases - for example, voting for someone else or hiring a different person.
It's easy to hypothesize that a generation of rejuvenated 200-year-olds could end up becoming a gerontocratic elite that maintains power over younger people, but how would this be accomplished, exactly? Maybe the older generation is rich and powerful, but unless we're talking about a totalitarian world in which the masses are intentionally kept ignorant and poor, younger generations do have fair chances to make positions for themselves. Power and wealth come from knowledge, and, these days, knowledge is more freely and widely available than ever before.
Yet power and wealth don't come only from knowledge; they also come from powerful and wealthy ancestors. If we didn't develop rejuvenation, certainly all the Scrooge McDucks of the world would die sooner than they would otherwise, but their power and wealth would go to their heirs, and so on over the generations, which wouldn't do much to prevent the creation of an elite. So, no, old age is not an easy way out of the problem of powerful elites ruling the world, and its absence wouldn't make the problem any worse, really. The only possible way out is giving everyone equal access to knowledge and equal opportunities. Inevitably, some will end up being more successful and thus more powerful than others anyway; however, if this allows them to become an oppressive force on the rest of us, I think this is a problem with our socio-economic system, not with the existence of lifesaving medical technology. I don't know about you, but I'm not very keen on waiting until the "perfect" society or "perfect" economic system are built before we decide to cure the diseases of old age.
I think fears of a society where rejuvenated elderly make younger people's lives more difficult are misplaced in that they assume present-day scenarios will exist in the far future. Take the concern about jobs, for example, rejuvenated old people would stick to their jobs forever and make it harder for young people to enter the workforce. It sounds bad, but there are a few assumptions behind it that we should question. First, would rejuvenated old people actually stick to their jobs forever? Why? You hardly hear of a professional who was in the exact same job for forty years these days. More broadly, career change is a thing already. After all, after forty years in the same line of work, it's conceivable you might want to try something else, thus making room for others to take your place. Will rejuvenated old people be allowed to stick to their jobs forever? Not everyone is a manager in charge of decisions, and your boss may well decide to lay you off, rejuvenated or not, and hire someone else.
I'd say it's rather silly to oppose rejuvenation today for the reason that, in a century or two, it might cause an unemployment problem due to too many people being alive. It's simply too long a time to make any even remotely accurate predictions on what the job market will be like or if there even will be any. In all honesty, I think it makes more sense to worry about a concrete problem that we already have today - the ill health of old age - than worry about a hypothetical one that might or might not happen in a hundred years' time. As time goes by, we'll have a better picture of potential future problems lying ahead, and we'll be in a better position than we are in today to do something about them.