Very little is certain in any aspect of genetics, but as I said before, the academicians and virtually everyone in the hobbyist community except people from the Balkans like Garrick and Miroslav believe that this particular lineage of I2a existed in more northern areas and became part of the Slavic speaking community there, only arriving in the Balkans with the Slavic migrations of early Medieval times.
It seems to an uninterested observer that people who have been pumped with "Slavic" propaganda since the 1800s and maybe even before just want to have it both ways. They want to be "Slavic", despite all the evidence that most of them are at least half and often more definitely not "Slavic", and at the same time they want to be "indigenous", so they go through these contortions trying to show that the only y marker which could possibly be "Slavic" is indigenous, which would mean that the Slavic languages are also "indigenous", which is patently absurd. For one thing, except for some ethnocentrist linguists from the Balkans who are mostly ignored, every linguist in the world knows that Slavic is a relatively recent language, and every archaeologist, anthropologist and geneticist in the world knows that there was a migration by Slavic speaking peoples into the Balkans after the fall of the Western Empire and the weakening of the Eastern Empire. It was just a slightly later later Barbarian invasion.
You can't just look at the arguments for this y lineage being "indigenous". You have to look at the whole pattern or story that is being sold. If this lineage is not "Slavic", where did all the "Slavic" ancestry come from? There's far too little R1a to account for it. So, it has to be "Slavic". It also has to be "indigenous" for some people, but that would require us to believe that it was already in the Balkans before the arrival of Slavic speaking peoples in the Middle Ages. In that case what the heck y dna did those people carry? Simple, for these type of people. We'll just pretend that all the archaeologists relying on data and all the historians relying on actual documents are wrong and that there was no "Slavic" migration.
The whole thing is utterly ridiculous. How even someone who has been brainwashed by an education system and a culture under the control of autocrats and fifty or more years behind the times can believe this illogical and a-scientific narrative is beyond me. Of course, this is the Balkans....
I very appreciate the work of the moderators of the forum that requires a lot of knowledge and a lot of concentration and congratulations.
Moderators can certainly be mistaken. First nobody can know everything which is true postulate and these are topics where there is no final answers because the situation is changing with each following findings.
What we now have from the standpoint of science are four theories:
1. Russian/Pan-Slavic theory
According this theory I-CTS10228 is Slavic marker. His presence in today's high concentration areas is consequence of Slavic expansion (5-7 century).
This theory you support.
2. Thracian theory
According this theory I-CTS10228 is Thracian marker. I-CTS10228 is present in genetic fund of Northern Black sea region/Romanian/Eastern and Central Balkan population before of a mass migration (3-7 century).
3. German/Western European theory
According this theory I-CTS10228 is German marker, the most represented among Goths and Gepids. It had more directions, coming to the Balkans in 3-4 century and spreading to the Eastern Europe where Germans and Slavs was overlapping. In these processes carriers of I-CTS10228 were Slavicized.
4. Illirian/Western Balkan theory
According this theory I-CTS10228 is Illyrian marker, indigenous to the Western Balkan.
Every of these theories has pluses and minuses.
For example one of the minuses of Russian/Pan-Slavic theory is that I-CTS10228 is founded much before Slavs entered the historical stage, Russian/Pan-Slavist scientists have no explanation for this fact. Minus Dacian/Thracian theory is that although no one can doubt that I-CTS10228 could be present in Dacian and Thracian genetic fund before 3 century issue is whether it could be present in large quantities. Minus German/Western European theory is although it can be well-adjusted with the assumption that most probably I-CTS10228 emerged in Central Europe (today's Austria, Germany, Denmark, Western France, Northern Italy, Slovenia, Czech Republic etc.) and it was presented at German people I-CTS10228 didn't spread in all areas where German tribes moved. Minus Illyrian/Western Balkan theory is that could hardly explain strongly presence I-CTS10228 in Northern and Eastern Slavic countries.
What is needed to solve which of these four theories is most appropriate. It is patience. We will see newer findings.
Personaly for me second theory (I-CTS10228 is Thracian marker) could be best adjusted with a numerous dilemmas including language and well geographically connect different areas.
But I don't advocate none of these theories as final. Therefore I have no idea where you concluded which of these four theories I prefer, I only criticized attitudes that due to insufficient evidence one of these theories is winner. No, it is not true. Practically, all of them are in playing.Another thing is how many probabilty, what is very changeable with new evidence.