IBM did not "go under" they just pretty much started and then got out of the PC-making business in the 80s. IBM still is a large company ($79 billion in revenue) involved in a huge variety of commercial markets. PCs were just a fraction of their business at the time and like any big company decided to pivot when their business models could not compete with huge competition of what used to be called "IBM clones". It used to be IBM's main product line was their "Business Machines" (when computers were only mainframes) along their service. Now they are pretty much all service, except for some experimental concepts like Watson.
On the other hand Apple is pretty much only in the PC and consumer electronics market. They don't have much room to pivot like IBM. They have had some troubled times, saved by the iPod, the iPhone, and iTunes (and Steve Jobs). Their income is is $229 billion, so I guess you can say Apple is winning businesswise. As long as people will pay a premium for their quirky UIs and they can predict the next big thing for consumers and be first to market, they will be around.
While IBM was not ultimately successful in the PC industry, the Intel/IBM computer design is pretty much the direct ancestor of most PCs today. Unlike the Apple II, the PC design was open enough and could be functionally copied without infringing on any of IBMs IP. Additionally, unlike Apple, IBM did not own the IP to the Operating System, Microsoft did. This opened the door for dozens, probably hundreds of lower cost (and sometimes higher performing) copy cat designs of the original IBM PCs to the point that IBM could not compete. Dell Computers is really about the only survivor of the "clone wars", starting his business in his College dorm room and somehow surviving it all.
Given the open bus architecture, Microsoft, Intel, clone makers, and add-on card makers pushed the PCs to evolve. Bus architecture evolution:
* ISA (Industry Standard Architecture) bus - created by IBM in the first PCs (7.9 MB/s)
* ISA+VLB ( VESA Local Bus) bus - VLB used for graphics and other IO cards (127.2 MB/s)
* PCI (Peripheral Component Interconnect) bus (127.2 MB/s)
* PCI + (Accelerated Graphics Port) AGP - the AGP slot was the start of 3-D graphics (1066 MB/s)
* PCI-X (PCI-Extended) - Higher frequencies and more bits (4,260 MB/s)
* PCI-Express - Current with recent standards able to achieve (15,750 MB/s)
A PC bus today, particularly something like a video graphics card has ~2k times the throughput of the PC bus in 1981 (37 years ago). Which makes sense of you compare an Intel 8086 with an i9-7980ex