Overwhelmingly their target base is women. If you look at their newer markets around the world, uptake is always dominated by that demographic. Pinterest doesn't release very detailed information on their user demograhpics, however I'd be surprised if it's not 2/3 women or greater.
I entirely agree with the annoyance of the Pinterest image mess in Google image search results, which has been going on for many years now. However there is a legitimate place for Pinterest in terms of what it does. The parent should primarily be annoyed with Google (which is intentionally - they're fully aware of what's going on - filling their image results with Pinterest images), not Pinterest.
I can't use it because of it's closed nature, but it has a very good set of image cataloging and sharing features. Add in the valuable network effects and I see why people use it for idea sharing/collaboration.
I understand frustration of Google image search users. But pins in google search results are Google's problem. I switched to DDG myself, and find it more straightforward for keyword based image search.
Their image related grouping works way better for the things I'm interested in (art/illustrations) than other sites.
I believe your anger is misplaced, and directed at the wrong target as well. It's Google Images offering the crap results, and I think you already know that. Reddit is also loaded with plenty of low quality content, as is Imgur, as is Facebook, as is Twitter; all large social networks are flooded with epic piles of garbage; Google's search results are clearly the problem re Pinterest images showing up in Google image search.
To answer your 'question,' the 416 million monthly active users very obviously aren't staying with Pinterest just because Google Images has a lot of Pinterest image results.
I don't like TikTok, it doesn't appeal to me. The very short video clips mostly annoy me. I still fully understand why so many people use it. Google indexes their video content - I wouldn't blame TikTok if Google's video search turns to crap over time due to that, I'd blame Google.
When I find an image on Google search and I want to go to the source site, I want to see that exact image there.
Not scummy paywalled "I show Google something else than you" images.
Plus it's pretty obvious that in many cases they scraped the original, which then they outrank in SEO.
That's unlikely to happen as Pinterest is currently booming. Their global usage is growing fast for an already large social network. They have $1.7 billion in cash, with little to worry about financially for the next five plus years. Pinterest is likely to get a lot larger yet given their growth rate, they're up to 416 million monthly actives with zero serious threats in terms of competition. Facebook's attempt at building a competitor failed nearly immediately. As with LinkedIn, they occupy a niche in social media that they entirely own.