Just a few observations.
When you look at old (1920's etc.) pictures of horse racing crowds, the grandstanders are donned in suits and hats. I don't see a lot of young kids in those photos.
And one could bet sports in those days....just check in with Arnold Rothstein. :). Corner bar books have been around a long time.

Today, at any big event, (Belmont Stakes, Derby, Preakness, Travers, etc. ), the crowd is inundated with young people.

Casinos and poker? At most casinos, they poker rake is 10% from every pot. A little bit more goes to their promotions (high hands, bad beat jackpots etc.).
Then, the obligatory tip to the dealer when you win the pot.
Poker tournament takeout is usually somewhere around 15 to 20%. No bargain.
Table games have lower takeout but you must play every spin and every hand. If you are not a competent blackjack or craps player, takeout could plausibly rise from the 1-2% to 10% or more.

Online poker? Most of the young players who started online will tell you they were involved they were being cheated.
All those big names (Lederer, Ferguson etc. ) were involved in poker websites that cheated the players and, then, when the gig was up transferred 10's of million dollars to foreign bank accounts. Many never recovered the cash left in Full Tilt etc.

I totally agree that horse racing needs to improve 'marketing' etc. but this is hardly a recent dilemma. I cannot blame it on poker or sports betting. (what does your bookie take?)

The big advantage the horse player has versus casino games is that you have the ability to select which race to wager your money. If you feel you have no edge or 'value', you can pass. When you judge the odds to be in your favor, you wager. At that point, if you correctly assess the 'return' on your wager, the odds should actually be in your favor. Thus, 'effective' take out (my phrase) is actually zero.

Au contraire, once one bets the roulette wheel, or rolls the dice, or have blackjack cards dealt, the house is the favorite. Over time, the casino takeout rules.
I suppose Poker is really the only casino game where you can overcome the takeout (rake). A player that is selective in choosing what pots to enter.(weak opponents, premium starting hands etc.) can structure bets at each street that favor him.

Probably stir the pot a bit with this post....

Regards

Bob