I can't speak for the designers' motivations, but here are some possible reasons:
- It's simple. Having one tempdb for everything is likely simpler to implement and simpler to configure.
- It works. A lot of the time the shared tempdb isn't a problem. When it is, your links provide some mitigation. If it still remains a problem, then you can use separate servers to resolve it and there's a decent chance you would need to do that for other reasons as well, i.e. the shared tempdb isn't your only performance problem.
- It's easier to manage. You can set one quota size for the tempdb rather than one for each per-database tempdb. Further, you don't need to try to predict and over-allocate how much temp space each DB would need. You can easily stripe and/or host the tempdb on a different disk as suggested in your links.
- SQL Server supports cross-database queries for which it is not obvious which tempdb should be used in your proposed scheme.
- It shares infrastructure, e.g. the log file.
My understanding is that early in SQL Server's life one of its major selling points was simpler database management relative to other products at the time (i.e. Oracle, DB2). My understanding is a large part of this accomplished by reducing and simplifying the configurability, e.g. by using algorithms that were inherently more adaptive, incorporating "auto-tuning" algorithms internally, or simply not providing the ability to configure certain aspects. I'm speculating here, but I also suspect tempdb contention was likely much less of an issue at the time than tempdb allocation. Having a single shared tempdb simplified management and made it easy to allocate a set amount of disk space for temporary data. Being shared, it would also allow any database to use as much of that temporary storage as it needed in a burst of work while not requiring allocating temporary storage for each database in accordance to its worst-case usage.
I could easily imagine a new, mostly from-scratch database (re-)implementation by Microsoft choosing to make databases more isolated and significantly eliminating shared resource to improve scalability/elasticity.