There are meant to be various resolutions out there, all moving us somewhere closer to an upper limit which isn't worth going beyond for a 35mm sensor. For most purposes, a 20 odd MP sensor is already good enough, but for those that want maximum resolution, where will it end up?
Even lenses which are razor sharp with current sensors between f2.8 and f11 are likely to start showing their flaws when the MP goes up. The current 20 odd MP sensors start to suffer from DLA (diffraction limited aperture, or softness due to the small aperture) at around f11 with any lens. If the sensor can has a higher MP count, that smallest aperture with full res drops down - the D800 has a limit of approx. f7.8 - and most lenses are at their sharpest at around f8 - so it takes a really good lens to get the best out of a sensor that dense. Canon are releasing some very good lenses just recently which appear to out resolve the current Canon sensors at f2.8, and will possibly out resolve 36MP at f2.8 - but you've still got a reduced useful operational window there.
Its a exponential curve, so according to my calculations a sensor which is past its optimal range at f5.6 has 70MP - f4 is 138MP, and f2.8 is 282MP. Even at 70MP, f5.6, its unlikely any lens will give 100% pixel sharpness at f2.8, and to even appear perfect at f5.6 will take some great glass. So to get the best out of that sensor you're probably limited to one or two lenses, at one aperture setting. Select any other aperture, and even if everything else is perfect (tripod mounted photo of a stationary subject), you're not getting full resolution. And the narrow DoF at f5.6 at those magnifications (why else do you want such a high res, other than to print massive and study it up close) largely renders the extra resolution useless. This is possibly taking it too far for any practical uses.
Any sensor intended to work well for video will ideally be a nice round upscale of the intended HD resolution. At 1x 1080p, filled out to a 3:2 frame, its 2.5MP. 2x, its 9.8MP. 3x, 22.1MP. 4x, 39.3MP. 5x at 61.4MP will not allow for a useful upscale of 4k, so such a high res is really only suitable for 1080p, and will likely be no better than 39.3MP. And 6x is 88.5MP - while it will downscale for video OK, it's too much for stills at its native resolution by my reckoning.
So is 39.3MP the ultimate resolution for a 36x24mm sensor designed with HD video in mind?
And is it as far as we need to go for stills, or will demand make some manufacturer take it higher still? Diffraction sets in at f7.5 at 39.3MP - not inconceivable for landscape and studio work, but would some users want more, considering the limitations it brings? If you need much more resolution to print at massive sizes and still look good when studying it with a magnifying glass, you're hitting the limits of the size of the sensor.