I never liked waiting on the fast platforms at St Albans, I completely agree, massive accident waiting to happen there. Maybe building a new terminating platform (s) could mean that the fast platforms are fenced off like many other station in the country?
But the problem is that the volume of passengers passing through St Albans City station, could not be handled by the slow lines alone unless there were no stoppers between there and West Hampstead Thameslink in the peak hours. That might be convenient for the remaining MML passengers sailing through platforms 3 & 4, but would sort of mess up travel for up to 3 million* season ticket holders at the intermediate stops. There are plenty of stations where trains pass through platforms crowded with commuters at high speeds so it would be more than a St Albans City specific risk.
* Assuming that most of those 'season ticket entries' in the estimate for station use are travelling south in the morning and north in the evening peaks.