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A decline in recruitment of novices is a whole other set of questions
At the level of our club, this is a major issue. In the last 3-4 years we've had half a dozen regular testers move away. West London population isn't going down but we've had precisely zero new ones join to replace them. And the loss of that critical mass means novices have far fewer role models so fewer have started TTing.
gamification of turbo training
This is a good point - can get competitive fix without going outside.
Some more than others.
Your point 0 is definitely as old as the hills, but shouldn't affect overall numbers. People in their 20s have always dropped out to concentrate on career and family, but they come back in their 40s and it's a zero sum game
Point 1 is only relevant at the pointy end, as long as everybody else accepts it. I think there has been a real philosophical change in the 21st Century where fewer people are happy to be also-rans - they would rather not compete than be beaten. This is a society wide problem, so it's hard for TTing to solve, yet TTing in itself is capable of addressing it by being one of few sports where it really doesn't ruin your day if somebody is 50% better than you.
Point 2 is more of a perception than a reality, but I get it, especially as many people who rode only for fun have retreated to the turbo cave during 2 years of lock down. Going out in traffic again seems as scary as the first time you did it. TTing is safer than commuting, because we make it so by picking our time and place, but the two things people are rubbish at are statistics and probability. Feels trump maths because homo sapiens doesn't have maths as bundled software, and feels were better than nothing.
Point 3 might explain a temporary bump (or reduced rate of decline) in TT participation in 2020/2021, so you'd need to look at 2015-2019 to see what the base level was without refugees from massed-start racing
Other reasons people give are the gamification of turbo training, which has made it an end in itself. People no longer need actual competitive cycling to motivate their training. Inflation has made people look twice at their hobby spend, particularly when it involves long car journeys. Rationally, petrol going up 60% isn't really a deal breaker for most testers, but it's enough sticker-shock to make them question whether TTing because they've always TTed is a good enough reason to continue.
That's just the people who used to TT and now don't. A decline in recruitment of novices is a whole other set of questions.