Even when all empires fall, It's important to remember that as long as humanity in general continues to discover, explore, solve and invent, the better off we all are. That's why the price of indices always tend to go up. Even when they become stagnant, they eventually keep on increasing. The only thing that can revert this is a total collapse of society, which is unlikely in the present moment.
Nevertheless, it's also important to observe the health of a trend. When price increases violently, then a correction is likely to occur. These corrections can be severe or simple technical resets. Technical resets are good for everyone as it allows new buyers to enter the market as well as provide good buying opportunities. However, circumstances can lead the price to not have reset but instead have a correction or a crash. The difference between the two is that a correction is slow to reach the bottom, while a crash is a sudden move downward.
As one can see here leading up to the 2000 dot com crisis the uptrend was quite healthy, and it did a slight reset before going into euphoria, where price goes into the 3rd standard deviation range while pushing price higher and higher, before price lost momentum and eventually had a correction as the uptrend was way too aggressive. Meanwhile, leading up to 2008 crash there is a very aggressive uptrend, completely breaking into higher level deviations without going through the stages of a healthy uptrend. Causing the price to crash once the market realized that the system was still heavily corrupted by greed.
However, since things throughout time do improve, this allowed for another aggressive uptrend to form which instead of running into a crash it went into a technical reset which latter became the longest bull market in USA history.