I wouldn't say it was really designed, except that satoshi designed this general sort of scheme (declining rate of mining) for Bitcoin, with only somewhat vague justification and most other coins copied it (with variations such as using a smooth curve instead of big drops). In the case of Monero, there was a bunch of discussion, some disagreement and some confusion, with the current curve being the one that was launched.

Ultimately the curve in Monero ended up being a bit more than twice as fast as Bitcoin (not counting the tail reward, which offsets somewhat in the other direction). Some people suggested that might be good because the crypto market is far more mature now, and moves faster (the first year or two Bitcoin was very obscure and the code immature). However, regardless of that difference, there is no way to know for sure that the Bitcoin, or Monero, or any other schedule is in any way optimal or even good.

My philosophy is you have to get the coins out there, and it is better to give some years for the market to absorb them in an orderly manner rather than have 1-14 (or even zero) day fastmines/instamines/etc. that are easily manipulated, but other than that there isn't really an objective basis to say that 4 years or 8 years or 20 years is best.

I also agree that part of the goal of mining is to keep the price low, in the sense that it makes cornering the supply and thereby deliberately driving up the price difficult and expensive. If you want to control a large portion of the supply instead of merely holding coins you need to continually spend money to mine them or buy them, otherwise your holdings are constantly diluted. When a mined coin goes up in price and manages to hold on to those gains, it is a pretty good indication that there is actual demand in the market, not just some small group of insiders holding most of it and refusing to sell.