Major overreaction by many (who think they know better), no surprise there.
There was an inspection and it was deemed safe to continue, bypassing the area of concern.
Could there be other undiscovered holes hidden below the surface? – possibly. This could be the case on any racecourse.
If we are to take seriously any of the paranoia exhibited recently, then all racing in the UK must be immediately suspended until a full survey of every racecourse has been performed, using ground penetrating radar.
Alternatively, we can take the sensible option and continue to trust the dedicated grounds staff with many years of experience, who inspect the track before, during and after each race meeting, to identify any areas of concern for further investigation and action if required.
Risk should always be properly assessed and mitigations put in place where appropriate, which is what Cheltenham did. It is never possible to eliminate risk completely.
....and you've got to look a long way back for anything else.