As I understand it (which is probably not correctly), the issues of torsional rigidity and rollover protection are separate (someone get engineered in here to explain it).
With modern engineering, you can design a convertible with equivalent torsional strength to a hard top, so a targa would also be build-able to equal strength (i.e. t-top no longer holds an advantage).
As far a rollover protection, I was of the impression that the fixed roll bar was the primary concern. As long as that held firm above your head, the car would land on the hood and roll bar, keeping you safe so long as your head was not taller than the line drawn between the two. Since the windshield will likely shatter, and its frame is usually heavily raked, it doesn't provide much resistance to being flattened along the roll bar/hood line. Without the windshield for a brace, the t bar is just going to push the center of the windshield frame down until the roll bar makes contact with the ground. Thus the t-top adds no greater rollover safety versus a targa. In fact, as convertibles now have popup roll bars of sufficient rigidity to support the whole car, neither the t-top nor the targa hold a sufficient advantage over a full convertible.
The final downfall of the t-top (and the targa), is that roof mechanisms (softtop, PRHT, retractable moonroofs) have reached sufficient reliability that you will likely never need to repair/replace them.
/start tangent
As mentioned, the unintended discovery of the targa top was the intermediate between coupe and convertible in terms of experience. They could both give you the open air atmosphere without the wind buffeting and the added complexity/hassles of the rag top. However, as Porsche (and tonyola) has discovered, this design goal lends itself to a moon roof (albeit a monster one). But hey, a brand name sells, and they have to sell something with the Targa name, they invented the whole genre (shutup Triumph! No one wants to call it a "surrey top").
That said, a modern 914 version better have a targa top.

More like a little of the open-air experience. A real droptop offers nothing but a windshield surrounding you – no back window, no C-pillars, no blind spots, nothing overhead but sky. A T-top doesn't even come close.