Dannihilator says:
Then how come AG2R used titanium tubed litespeeds for awhile then?
They used the Titanium framing, thinking that the lightness of the frame, would compensate for the degree of flex. However, it didn't work! Eventually, it became evident that Titanium just wasn't a good competitive racing frame material.
Regardless of frame material, frame construction has evolved over the years. There's a member here who had a custom all mountain hardtail made for him, where the tubing material was titanium. The frame lasted a reasonable time given the beat down it took over that time period.
Titanium is very durable. However, you most probably wouldn't want to compete in any DH MTB races with it. Sure, it would most likely survive the task. You just wouldn't win any races, due to flex and efficiency issues.
Reference:
www.bikeradar.com/news/article/spin-spitfire-mkiii-titanium-race-bike-just-in-31435/