People are entitled to be disappointed with their grades whether that is an A or an E. Yes those threads are perhaps not in the best taste to read when you received a lower grade but success is relative.
I was disappointed in my A-levels when i received an A rather than an A* in product design, looking back on it it was silly but the emotions were valid at the time as i felt i had not performed to the standard i could of (i had a clash that day and had to sit product design directly after doing a huge history exam).
The point is people are just as valid to be disappointed with a grade someone else would consider good, everyone has different goals for themselves. If someone has worked incredibly hard to get a C they should be allowed to feel proud just as if someone has worked really hard and felt their A wasnt a true reflection of their effort.
Yes asking if 10 '9' grades at GCSE are good enough for oxbridge is a bit silly and the question could be answered easily with research, but these are often young people and in a lot of schools there is the mentality pushed on students that anything less than a certain grade is a failure. Yes some of these threads are trolls but some are genuinely concerned perhaps slightly naive students.
TSR is also not a representative sample of the population, naturally people who care about their studies are going to be drawn to a website where you can receive study support.
To be completely realistic it is not down to the poster of a thread how that thread makes you feel, you do not have to read such threads and if people have a query they should be allowed to voice it no matter what grade they obtained. Anyone is welcome to ask for advice on TSR.