people seem to be increasingly reluctant to engage in physical activity particularly when that involves expense)
I can only speak for the US, but I think you might be 180deg out of phase.
Skiing has has record years in terms of gear sales, accessory sales, number of participants, and number of participant days over the past few years. In fact skiing's best year ever was the same as diving's low point over the last twenty years.
Similarly, cycling has rebounded in the past few years... particularly in the higher-end road and mountain segments.
Paddle sports is growing.
Even golf is making a bit of a comeback.
I actually think that part of the issue for diving is that people are TOO willing to participate in physical activities that are expensive... to the point of distraction. Cycling this weekend, SUP next weekend, training for a triathlon, kid's travel soccer tournament weekend after that, camping, climbing, hiking, trail running, etc.
REI - a very high-end outdoor sports chain in the US - is continuing to expand into new markets as each year is a new record for sales.
Many people - who were, or could have been divers - simply don't have time to dive because they are doing too many other expensive physical activities.
The innovation that the scuba industry requires needs to start with a better understanding of the audience's wants, needs, desires, and barriers.