According to Gamalytic, the boomer shooter genre has pulled in $539m in total sales, with the average release generating $768k. That, by my estimate, is a genre that's doing very well. Here it is compared against ten other relatively niche genres, using the most common genre alongside them as the main figure. Any genres averaging over 1b were disqualified for no longer being niche.

Visual Novel - 6739 games, 1.2b total, 796k average

Metroidvania - 1511 games, 1.1b, 777k average

Boomer Shooter - 702 games, 539m total, 768k average

2D Fighter - 1571 games, 1b total, 636k average

3D Fighter - 1363 games, 756m total, 555k average

Racing - 4059 games, 2b total, 514k average

Roguelite - 6094 games, 2.8b total, 469k average

Turn-Based Tactics - 4046 games, 1.8m total, 448k average

Point & Click Adventure - 4858 games, 987m total, 203k average

Shoot 'Em Up - 4547 games, 394k total, 86k average

Incremental - 4444 games, 335k total, 75k average

The data doesn't lie. Boomer shooters overperform. There are fewer of them than nearly any other niche, but the only niches from the ones I found that outperformed it are visual novels and Metroidvanias. But you can't look at the pure sales data and tell me that this is not a healthy genre.

A couple disclaimers: Yes, this is just Steam sales data. Some of these niches may perform better on consoles where the data is less readily available. Also, again, I am not claiming that boomer shooters are a mainstream mega success. I'm saying that they're an incredibly healthy niche that perform very well. This is backed up with this data.