Whilst I'm happy to be corrected, I'm going to be boring and say health and safety. From the Health & Safety Executive in the UK:
The law says that all demolition, dismantling and structural
alteration must be carefully planned and carried out in a way that
prevents danger by practitioners with the relevant skills, knowledge
This guidance includes the following:
Workers and passers-by can be injured by the premature and
uncontrolled collapse of structures, and by flying debris.
A safe system of work is one that keeps people as far as possible from
the risks. This may include:
- establishing exclusion zones and hard-hat
areas, clearly marked and with barriers or hoardings if necessary
- covered walkways using high-reach machines
- reinforcing machine cabs
so that drivers are not injured
- training and supervising site workers
It seems a safety net to protect workers would be a pretty understandable extension of this guidance.
Boring answer, I know, but as I can't find any plot driven reason it would be there, I'm going to say health and safety saved the day.
Of course, we don't know what will happen in future films yet (and Mendes isn't giving anything away). Not only was their spray paint on the wall, there was also a conveniently located power boat as well. Perhaps Blofeld really did want to encourage a chase, or encourage his nemesis to come out. But given how quickly Bond pinned him down, I'd argue the boat was just there and the markings was either random or done before the net was installed.