Let's see, what are my sources? Every single resource on the internet that talks about Korean phonology, and my own personal observations. I can differentiate aspirated consonants without problem, and its overly obvious that the younger generations aren't bothering to differentiate between aspirated, doubled, and tenius consonants. This is particularly obvious in videos where you have an older speaker and a younger one talking. The older person's pronunciation is far more pristine than that of the younger person, who honestly sounds quite sloppy in comparison.
And besides, I've been watching k-pop for hours everyday since Gangnam Style came out. And its pretty easy to find the lyrics for songs online, even in the original hangeul. They don't even pronounce the names of the songs right 99% of the time, and when they do its probably just by accident.
Also, this collapse has been going on for a while. Its common for the tenuis consonants to be lighly aspirated at the beginning of words. Actual aspirated consonants have noticeably heavier aspiration. Also, if you pronounce the secondary articulations right, then syllables will have a low, medium, or high tone depending on which consonant they begin with. For instance, syllables that begin with an aspirated consonant take on a high pitch. This is a natural consequence of the aspiration. And btw, even my sister can hear the difference in tone, even though she's monolingual and certainly NOT a linguist. Getting back to the point, this is causing Korean to develop phonemic tone, which lines up with research on how tone developed in other eastern languages.
Also, research conducted by André-Georges Haudricourt shows how tone developed in Vietnamese. He compared it to a related language which lacked phonemic tone. He found that words with the same tone in Vietnamese typically ended with the same consonant when translated into the other language. He deduced that tone must have developed to help differentiate words with final consonants, because humans have a tendency to cut off final consonants. Because the tone was easier to hear than the final consonants, the final consonants were simply lost, leaving a language with phonemic tone and a limited list of consonants a word could end in. He deduced that the tones in Chinese must have developed the same way, since it shows the same features of phonemic tone with limited syllable coda. Also, its obvious that Korean is currently undergoing the same process, except its causing their initial consonants to fuse together. Note that the difference is only made at the beginning of syllables, at the end of syllables most consonants assimilate to a p, t, or k, often with no audible release.
Anyone who's trying to learn Korean would realize this sound change, because its so overwhelmingly obvious. And I've literally been listening to and trying to follow along with k-pop songs for years. And yes, I can make out the words of a language I don't know. I once heard a German song long ago that I liked. I didn't know a word of German back then. Years later (as in at least 5 years later), after I had learned German, I was able to recall the song well enough to translate part of the chorus, even though it was gibberish to me at the time. And I was right, because it helped me find the band that song came from and I was able to buy the album. Obviously, I've always been very attuned to phonemes. I've never even been able to understand how anyone could struggle to hear the difference between ʃ and ç. I mean seriously, if you can hear the difference between t and d, then you should be able to hear the difference between those two consonants. The only things I've really struggled with was learning to do the alveolar trill (took me a year to figure it out), and learning click consonants, but the latter was just because I was having a hard time finding resources on it. It wasn't even until a few years ago I was able to find a recording of what they sounded like (though by that point another phonologist had described to me how to do some of them, though the only ones I could do was the bilabial and alveolar clicks).
I'm suspecting I could get away without learing the tense consonants. For one, they're not clearly differentiated anyway (at least by k-pop idols), and besides, I'm studying Korean more so I can get a more in-depth knowledge of an agglutinating grammar, I don't really care if I'll actually be able to speak the thing in the end or not. Not like I can afford a trip to Korea anyway. And the sole reason I chose Korean for this, is that I watch k-pop everyday so I figured that would give me some encouragement. Before I was actually considering Hungarian for an agglutinating language.