Whats so terrible about pulling #2 tooth after failed root canal and just leaving it out?

Missing teeth: The bone in the area that the tooth was may diminish in size.. Other teeth may start to shift into the area of the gap. There is an increased risk for gum disease.

Integrity: Tooth loss = loss in arch integrity. It's called posterior bite collapse. It's like losing one stave from a barrel and watching the barrel fall apart. Adjacent teeth tip and drift into the space while tooth in opposite jaw super-erupts, which enhances instability and interferes with normal occlusion/function. Additionally bone in extraction site will resorb. Replacement cheaper than the cosequence