Thanks for the questions. These two lines of argumentation are quite common responses, and I would address them as follows:

1. It is entirely possible that existential risk mitigation (e.g. AI safety, nuclear, biosecurity) is by far the most effective cause area in EA due to the 'Pascal's mugging'-style argument you put forth (i.e. the potential for trillions of future lives to be saved). If you believe that 100% of EA funding should support ex-risk causes then you will be unlikely to be persuaded to donate to anti-aging research. 

However, if you think there is also value in short-term cause areas (e.g. global poverty), given they have the advantage of direct, immediate and sometimes quantifiable return on investment (i.e. guaranteed 'bang for buck'), instead of only a possible chance of impacting the long-term future, and you support a more 'diversified' portfolio' in EA, then there is a case to be made for anti-aging. There is a trade-off here between potential impact (high for ex-risk, low for short-term cause areas) versus the probability that donations actually make a difference (potentially low for ex-risk, very high for short-term cause areas). 

Now, anti-aging falls between short and long-term cause areas on this spectrum - it is potentially much higher impact than short-term cause areas, but the feedback cycles and return on investment are slightly less quantifiable. That said, based on the preliminary models that I cite in my talk, even in a conservative case in which it costs one trillion dollars to bring the anti-aging technology forward one year in time (irrespective of whether this occurs tomorrow or in a hundred years), it is still better to donate to an aging charity than a GiveWell one, given the QALYs saved. Remember, the model assumes that this technology will arrive at some point in the future (if we are not wiped out due to an ex-risk, of course), so the benefit of donating in QALYs is based on the difference in how much sooner this technology comes, due to the lives saved who would otherwise have died due to aging in that time.

An extinction event would indeed remove all of this benefit, so it becomes a question of weighing up the probabilities - but this is already the case in the argument for donating to short-term charities like GiveDirectly versus long-term ones like MIRI. Interestingly, another way to look at short-term cause areas versus anti-aging is, all the 'benefit' from saving a person from dying to due poverty is lost when they die due to aging.  

If you are bullish on anti-aging (which, familiar with the science, I am) then this may lead you to favor anti-aging over ex-risk charities. If you think anti-aging technology is unlikely to come into the world for 1000+ years, or until AGI, then you may be inclined to favor ex-risk cause areas on this basis. Timelines are important here. While Metaculus polls suggest timelines for AGI are fairly short (strong AGI by 2055) they are even shorter timelines for anti-aging ('culturally significant development in aging research' by 2030).

2. The key part here is as long as they are happy. Currently, the status quo of human life is dying at ~85 years and spending much of the last 10-15 years of that time with significant physical and cognitive impairment, often including the onset of multiple co-morbidities (e.g. cancer, heart disease, type 2 diabetes, neurodegenerative disease etc.) that cumulatively account for 30% of all DALYs globally. 

Relevantly, the highest rates of depression, and suicides in men are among those aged over 70 years. In fact, the rate of depression in those aged over 70 is almost double that of those aged 50 or below. This is very likely the result of the social (i.e. being unable to live independently and without care), emotional, cognitive and physical impairment associated with aging. 

Remember that the goal of anti-aging is healthy lifespan extension not just life extension. In fact, some researchers think that health-span extension is more likely and/or a more important benefit of anti-aging research than lifespan extension. 

So in reality, the situation is whether we want a world of people who die at age 85 with substantial suffering in the last 10-15 years of their lives, or live to 100 (or more) with a 'squarer' mortality curve i.e. maintaining healthy lifespan for longer, and dying quicker - which centenarian studies have demonstrated is the corollary of slower aging. 

An alternative means of lowering the suffering associated with aging would be to euthanize people at say, age 45 - an age at which the burden of aging is relatively low. However, this would be morally abhorrent for obvious reasons, and hence anti-aging and 'squaring the mortality curve' is a better more attractive option for radically reducing the immense suffering in the world associated with aging.

I answered you here:

1. Yes, this is probably true. But see longtermist considerations of effects of anti-aging research. They might be in the same ballpark. Or not.

2. There are three ways in which the impact of anti-aging research is evaluated: DALYs averted and other short-term considerations, LEV being brought closer in time, and effects relevant to the long-term future. All three don't suffer from this objection.