Just plaster cast and then live with instability or pain. Earlier there were not many options. Even biddest of disabilities looked normal and acceptable. Today options are available , that too minimally invasive . And you I guess have accepted the advancement in technology.

ACL tears were treated conservatively in most parts of the world however high demand patients and athletes at least in the west used to undergo open surgical procedures( very few returned to Sports even after that). Now with the advent of Arthroscopy and minimal invasive surgery the same procedure can be provided to everyone. Especially young and physically active patients( return to sport now is very high). ACL deficient knee is unstable especially in twisting and cutting movements which are needed for not only for sports but also activities like dancing, boarding a crowded bus etc.. Also, unstable knee puts other structures like cartilage and meniscii at risk of injury. I suggest you see a sports medicine Ortho or a knee surgeon in your area for further advice.

Basically without Acl reconstruction patient can’t walk run and climb stairs so traditionally patient don’t do sporty activities and avoid stress on knee Thts why they develop early arthritis