There are various kinds of "first class", ranging from the regional first-class train carriage, where the sole benefit of first class is that nobody will pay anything extra for it, which results in a guaranteed quiet journey, to the Singapore Suites, with lounges (or even terminals!) separate from the business class ones, and food to rival the best restaurants on the ground.
The reality is that very few people use the latter type of service, and that's evidenced by the fact that most long-haul flights don't seem to have a first class cabin; for example IB, KL/NW, SU, DL, AY, and OS all have decent long-haul networks but do not offer any type of long-haul F service. Of those who do fly in F, there will be some top corporate people together with the odd "regular" business traveller who absolutely has to be somewhere but can't find a cheaper alternative. Ticket paid by others. Then, there will be leisure travellers on miles, plus a couple of upgrades if business is full. Ticket covered by airline. Those who actually pay the fare will be some people who want to make a special trip (e.g. honeymooners) as well as those who don't want to be seen (consultancy owner by competitors or international celebrity by a lot of nosy seat neighbours).
Sure, there will be a few "regular" wealthy people just paying more to travel in the most comfortable way possible (but aren't rich enough to buy/charter a whole plane), but they will be in the minority IMHO.
Personally, I haven't flown in proper First and am not really anxious to do it anytime soon. I do like some of the ground benefits (e.g. luxurious lounges and limo transfers). but parts of the inflight service which others seem to value (such as champagne, never-ending meals, personalised service) don't mean anything to me.
If anything, I prefer walking to the galley and asking for a glass of water to the obnoxious routine of using the call button, waiting for someone to appear, being addressed by name and generally in an "approved" way, ordering and waiting for the FA to go back to the galley and bring me the bloody glass.
Business class is great because it usually means breaking free from the prison that WHY is, and arriving somewhat rested and having eaten a meal hopefully better than a Rat Risotto. The difference between the angled-lie-flat seat and the (3cm wider) "extra wide flat bed" or that between a rump steak and a fillet mignon don't really make you more productive or in any way justify an extra €3000 on top of the business class fare. IMHO, privacy is about the only serious benefit that F has over J. If you aren't concerned with it, don't fly in F. I don't.