I think 762 can be beaten. I am not sure 73 will be, though.
If someone starts their career exceptionally early, like 18 or 19, and they stay healthy and consistent over a 20-year career, I think it could happen. They'd need to throw in several seasons where they get 50 homeruns, but it's possible. Bryce Harper has 43 homeruns in two seasons. If he has a good five-year stretch from say 25-30 where he gets 50-55 HRs each year... well, it could happen. I'm not saying it's likely that Harper or anyone else breaks it, but it's definitely possible. Especially if they play at a hitter-friendly park that suits their strengths. I think as conditioning and medical treatments advance, players will have longer, healthier careers, too. It might be another 50 years before someone breaks the Bonds record, but it can happen.
However, 73 in a season may not be possible. Well, it's possible. I think if an excellent power hitter went up to every at-bat with the specific purpose to hit a homerun, they might have a chance to break it. They would be hampered, though, in that I think that approach would result in a LOT of failure. That guy might hit 74 HRs, but he might also have a .200 average. So, he'd be fighting being benched and being detrimental to his team as much as the difficulty of hitting 74 homeruns.
I think at least some of Michael Phelps' Olympic medals records will ever be broken (he has records for total medals, total gold medals, most gold medals at a single Olympics, most gold medals in individual events, most medals of any color in individual events). This is especially true if he actually manages to win a couple more in Rio. He has 22 medals now, breaking the total medals record by four. But 18 of his 22 medals are golds -- the next-highest is 9. He won 11 of his golds, plus two of other colors, in individual events. If someone were to ever break his records, it could only come in a few sports that have a lot of different medal events and by an athlete that can maintain elite status for 12-16 years. I don't think it can be done.