which allowed the user to choose freely between 3rd party applications in opening data files.
and (from a comment)
I was under the impression that early PCs bundled their own applications (like word processor or spreadsheet program) without any means to have another application be used in its place (or documented means?). Was this the case? If so, what was the nature of the change toward allowing users to grant third party applications the same privileges? Thanks
The Answer is simply every OS and every PC.
Back in ye good ol' days OSes were way too primitive to even try something like a closed system. They where merely loaders to start some application.
Early microcomputers were complete open systems. Home computers and later PCs even more so. The first such were bare hardware, delivered without an OS. If at all, only a monitor program was supplied – sometimes as small as 256 Bytes, offering nothing more than a chance to examine and change memory locations. The Altair for example was delivered completely without anything. Programs had to be entered bit by bit via switches. Not too much of a hassle considering that the main memory was just 256 bytes.
Any OS would have to be either user written, or bought in addition. Even more so applications. While home computers like PET, TRS-80 or Apple II became soon more lavish, they still only supplied some frugal (by today's standards) monitor and BASIC as high level language and OS. OSes more like today only became available after disk drives where sold. Still manufacturers did only offer a DOS and, if at all, a very limited selection of software, mostly restricted to languages and tools. Or in case of home computers like Atari 800 maybe some games. The majority of applications where third party.
Similar there where OSes for each of these machines by third party suppliers. Usually due the fact that the original DOS was rather limited (*1). The market was quite open, and maybe except for some strange people no one could imagine what a closed system would be good for. In fact, even the idea of bundling was not much thought about.
The first (successful) machine with an application package included was the (professional) Osborne 1 of 1981. Here wordprocessor, spreadsheet, database and BASIC was bundled with an OS. Still, the OS was CP/M, an open, manufacturer independent OS, open to any kind of third party application.
Another kind fitting your idea would be early handheld LCD portables like the Kyotronic 85 family (Tandy M100, Olivetti M-10, NEC PC-8201, NEC PC-8300). They came with applications preinstalled in ROM - much like todays phones. Again, it wasn't about convenience, but the convenience of instant on usability. They could as well install third party applications in RAM - some even got user accessible ROM slots to install third party ROM based applications.
Without being open to third party software, computers might not have been the success they are at all.
*1 - Check for example NewDOS for the TRS-80