I went with ETFs sometime in 2001 or 2002 while I was sitting out the first crash. I had been in mutual funds since 1984/85 and had made a bunch of money. I felt good that I had been able smart/lucky enough to avoid the brunt of that crash and was wondering what I'd do if this happened again someday? In my readings I discovered things like moving averages and long term market timing. I knew that mutual funds in general, don't like it when people sell too much all at once. Even the long term signals can lead to whipsaws and didn't want any problems with mutual funds admonishing me to buy & hold etc. That's when I discovered SPY, QQQ and the like.
All the 'historical numbers' and diversity of indexing with the option to easily side step big down drafts and catch big up drafts without having to annoy other fund investors or managers and complicate their lives.