The competition with Casio may be one reason that we see portamento introduced in the next version of the top level PSR-E keyboard (PSR-E473, or whatever it may be called). It's not just in India. Here in the US, we can buy a $180 Casio keyboard that now includes portamento on certain synth voices -- the CT-X700.
In the past, there has been a kind of give-and-take tradeoff between the higher-level entry keyboards from Yamaha and Casio. For example, the previous high-level Casio (the CTK-7200) had organ drawbars, Leslie speaker simulation, more registrations, and the ability to modify a sound and save it to a custom voice, not just a registration. But the Yamaha has live-control knobs to make it easier to customize sounds and the ability to alter the sound during a live performance; the groove/DJ features; a sequencer that, while having fewer tracks than the Casio, has the ability to record both the main and dual voices (if the previous Casio sequencers were anything like those of the sequencers on the new CT-X models, most of the tracks could only record the main voice of a set up); and in my opinion, a better key feel and sound quality. And, also subjectively, I believe Yamaha has a much more straightforward and easy-to-use control layout and user interface.
By adding portamento, it really starts to get people thinking more about the Casios, and I would hope that Yamaha sees this and adds this feature in future models. Would this cause it to bite into PSR-S sales? Maybe a little, but the PSR-S keyboards have a lot more than just portamento, as they have more polyphony, more registrations, more registrations per bank (requiring fewer button-presses to change sounds during a performance), more sounds and styles, and things like vocal harmony in the upper-level models. By the way, does the PSR-S670 have portamento? I wasn't aware that it does, but then again, I never fully researched it, either.
Plus, Yamaha has put features in the PSR-E models that were not yet even introduced on the PSR-S models, such as the fact that live control knobs were on the E keyboards long before the S models, and the EW provides 76 keys, whereas no S-model currently offers 76 keys.