Can a "site split" cause a drastic organic search decline?

[ChristianMKG]

Let's say you have a client. They have two big, main product offerings.

Come early April of this year, one of the product offerings decide to move their product offering over to a new domain.

Let's also say you had maybe 12 million links in your inbound link portfolio for the original domain. And when this product offering that split opened their new domain, they 301 redirected half of those 12 million links (maybe even 3/4s) over to their new domain.

So you're left with "half" a website. And while you still have millions of links; you lost millions as well.

Would a ~25-50% drop in organic traffic be a reasonable effect? My money is on YES. Because all links to a domain help "rise" the page authority sea level of all URLs of the domain. So cutting off 50-75% of those links would drop that sea level a somewhat corresponding amount.

We did get some 301 redirects that we felt were "ours" in place in late July... but that really accounted for 25% of the total amount of pages with inbound links they took originally. And those got in place almost 4 months after the fact.

Curious what other people may think.

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[MattAntonino]

My first instinct is obviously to say "if you lose 50-75% of your links, will your rankings drop" - well ... yes? I mean that seems fairly obvious?

But I didn't want to be that obtuse so I thought about it a bit longer. And really, the question is much more teachable than that over-simplified answer.

What you are talking about is domain-level vs. page-level link factors. ANY link to your site affects the whole domain (domain factor) but a direct link to an inner page has more page-level effect. If you look at the Moz Ranking Factors Survey you will see this:

Domain-Level, Link Authority Features: Based on link/citation metrics such as quantity of links, trust, domain-level PageRank, etc.

And that, right there, is your answer. On the domain-level, we assume quantity of links, trust & PR of those links affects the domain as a whole. If you have poor quality links it can also tank the whole site even if they all direct to one inner page. So yes, the long answer is "quantity and quality of links can affect domain-level metrics and thus affect your rankings."

[EGOL]

Even though the terrain is different, battle on the web and battle on the ground have their similarities.  Generals on land have always use "divide and conquer" and "unite the clans" as effective strategies for victory.

If you chop a website into two websites then you divide your link popularity and divide your domain popularity.   You also eliminate opportunities for cross-selling and larger shopping carts in checkout.  Visitors may spend less time on the site, there are half as many reasons to talk about it or link to it.

I can think of lots of good reasons to combine websites.  I can't think of many (maybe any) good reasons to divide them.