The old system can't be be turned off but the web pages will become progressively unavailable (forwarding) as the new site develops. Seems only right, to me.
If I was responsible for maintaining the old system I might agree but as I'm not I don't.
The success of the new system will be measured by its takeup. Turning off the old system for no good reason negates that.
Fact is we're going to be dependent on the old system for event entry and admin purposes for quite some time to come.
So discussions about turning off the old system are beyond premature.
In the real world systems set to be superceded are generally allowed to whither on the vine or at least proper notice given to the point where withdrawal is simply not an issue.
Regardless of whatever approach is taken, all of this might have been thought through, consultation taken and appropriate guidance offered.
(For example, some process for reporting issues arising and the programme for addressing them would have been helpful...)
As is, we have a project which has cost x man years and £100k+ released apparently with no notice or preparation.
Not so much a soft launch as a humpty-dumpty launch...