You're right about needing a crystal ball. I don't think the doctor was lying to you. Some have MS for 20+ years without significant disability.
Firstly, I hope you don't even have MS, so all of this is irrelevant to you. Secondly, I don't believe you should worry about how you'll be doing 20 years from now because I think MS will be cured by then.
Below is an abstract of a study published last year that found MS disability progression happens more slowly than was previously believed.
I hope you hear good news at your neuro appointment.
Disability progression in multiple sclerosis is slower than previously reported.
Neurology. 2006 Jan 24;66(2):172-7.
Tremlett H, Paty D, Devonshire V.
Department of Medicine (Neurology), University of British Columbia, Vancouver, Canada. email@example.com
OBJECTIVE: To investigate disease progression and risk factors in a large geographically based population with multiple sclerosis (MS), using two different inception points--clinical onset and date of birth.
METHODS: The authors reviewed a database of subjects with definite MS and symptom onset prior to July 1988. The main outcome was sustained progression to Expanded Disability Status Scale (EDSS) 6 (requires a cane), using the date of birth and date of MS onset as inception points in separate analyses. Risk factors examined were sex, relapsing vs primary progressive course, onset age, and onset symptoms.
RESULTS: The study included 2,837 patients, followed prospectively for 22,723 patient years. The median time to EDSS 6 was 27.9 years, 15 years after onset; only 21% reached EDSS 6, and by age 50, 28% required a cane. Men progressed 38% more quickly than women from onset (p < 0.0005), yet both required canes at similar ages: 58.8 years for men and 60.1 for women (p = 0.082). A younger onset age predicted a slower progression, but those older at onset were consistently older when reaching EDSS 6. A primary progressive course predicted a more rapid progression from both onset (p < 0.0005) and birth (hazard ratio = 2.7 [95% CI: 2.2 to 3.3]). No onset symptom consistently predicted progression.
CONCLUSION: Disability progression in multiple sclerosis (MS) accrued more slowly than found in earlier longitudinal studies.
The authors also challenged two fundamental concepts in MS, demonstrating that neither male sex nor older onset age was associated with worse disease outcome.