The problem with powered subs that are attached to the mains is that they can't be physically separated from the mains to achieve good sound. In most rooms, subs need to be placed in different physical locations than the mains in order to sound good. Subs that are attached to the mains are almost guaranteed to sound worse than external subs, regardless of the quality of sub.
If you are comparing two subs attached to stereo speakers vs. two unattached subs that you can place at the midpoints of adjacent walls, then what you are saying has strong merit. But when you made this comparison, you changed the context. The previous comments were concerned with the pros and cons of a single subwoofer vs. stereo speakers with full-bass capability built-in.
I am befuddled by your assertion that you cannot achieve good sound with a pair of subwoofers integrated into the stereo speakers. Some of the very finest speakers that have ever been made are speakers of this type. The Mythos towers that Definitive Technology made were
phenomenally good speakers, for one example. And the speakers that Infinity made according to this recipe likewise sounded amazingly good.
When a pair of subwoofers are incorporated into the stereo speakers, they do in fact achieve a useful degree of mitigation of room modes (i.e., the beneficial effect that you identify as being of strong importance), especially compared to a single sub in
any single location. As such, the effect that you pointed to is genuinely an advantage of having full-bass capability in each of the two stereo speakers as compared to a single subwoofer. It would have been more genuine for you to have pointed this out rather than to have obfuscated it.
And there is again the fact that when the stereo speakers have full-bass capability built-in, this generally means that each of the higher-frequency drivers will be smaller than they otherwise will be, thus promoting broader dispersion and also making it easier to achieve directivity match at the crossover points. This is a manifest and substantial advantage of this approach. I don't like directivity mismatches, or the absence of high treble in the off-axis response, or the lack of good dispersion throughout midrange and treble. Any technique that is helpful in these related areas is akin to using more drivers, and is a useful technique in my individual opinion.
The advantages of having full-bass capability incorporated into the stereo speakers are substantial, not only as compared to a single sub as per the original context of the question, but even (albeit to a milder degree) when compared to a pair of separate subwoofers that can be placed anywhere in the room. In addition to the aforementioned advantage of better off-axis smoothness and dispersion, stereo speakers with built-in full-bass capability are far more easily accommodated by a typical listening room, vs. a couple of subwoofers that are placed somewhere along the walls, which is so awkward as to be patently infeasible in a typical room. Even in a large room it is an obstacle to have a big subwoofer sitting on the floor against a wall. In most rooms at least one wall has to be allocated to traffic, another to a sofa or a couple of chairs, and another to the stereo speakers and whatever else goes along with them, and usually there is other furniture than needs to be placed on the fourth wall if there even is a fourth wall and it doesn't have a fireplace. Stereo speakers with full-bass capability built in are somewhat more difficult to place in the room than small towers and some stand-mounted speakers, but the incremental increase in the difficulty of room placement pales in comparison to the challenges of placing two subwoofers elsewhere in a typical room, even if it is allowed to put them in the corners, which may be worse at mitigating room modes than for them to be integrated into the main stereo speakers.