Sony's a monster who never fails to sell less than 100 million units per cycle.
PS3.
Anyway tho, yeah, I don't see any of them going 3P. OP, it's not actually about how many consoles you sell, but how much revenue and, more importantly, profit, you can generate from your console ecosystem. That comes from a combination of hardware sales, 1P software sales, 3P software sales, subscriptions & services, and the prices for those as well as what volumes are sold at what prices, when they sell, and what the platform holder's cut is from those.
Weigh that against incurred debts, expenses, taxes, salaries, R&D, software development & marketing costs etc. that have to be paid (and thus deducted from those profits), and you eventually get left with a net profit. If the combination of revenue, profit and net profit aren't high enough to cover various expenses or have the company's market value increase in ways satisfactory to shareholders, then that's when they begin to run into trouble.
SEGA was operating under a mountain of debt and could not spin around large enough quarterly profits big enough to offset incurring losses. It's really that simple. Continuing to produce hardware for them would've kept bleeding cash due to the R&D, production, assembly, QA testing, distribution, marketing, customer support etc. costs that come with console production, so they cut it short before they truly ran out of money and would've had to declare bankruptcy.
Nintendo, Sony and especially Microsoft are not in anything near that type of financial situation, so they will be fine.
From a purely numbers and logistic POV, I guess I'd say Sony is the most "at-risk", but that's only if they had some catastrophic disaster with the PlayStation brand, which doesn't look to be happening. The reason I say this is because they generate nowhere near the revenue or profits of Microsoft, and don't have cash reserves to the size of Nintendo.
While they're a lot more diversified as a corporation than Nintendo, most of that is within consumer entertainment fields, which are inherently fickle and risky. FWIW gaming itself falls under that, but that's really the ONLY such type of market for Nintendo, while Sony is in multiples of them be it games, movies, music, television etc. Each additional one increases the overall risk. Unlike the majority of Microsoft's various market segments which come with lots of long-term security/rapport even if they are comparatively conservative/safe (business enterprise, data server networks, military contracts, medical etc.). The only arguably conservative/safe sector Sony has a foot in would be their financing services department.
Again, tho, we're talking some absolutely monstrous, catastrophic blow to PlayStation as a brand that would put a company like Sony at risk of folding out of position as a platform holder. I don't foresee that happening anytime soon.
