I feel it's a waste of money upgrading from one Canon rebel to the next rebel or even a 60D to the 70D.
I suggest that you upgrade only to a larger sensor body.
It is just a waste of money to buy another camera having the same size sensor IMO.
Not at all true. The difference between, for example, an original Rebel and even the venerable XTi is significant (6.3 MP vs. 10.1 MP). The difference between the XTi and the 70D can be significant (10.1 MP vs. 20.2 MP) if you're trying to do significant enlarging, though only if there's enough light to not require massive noise reduction.
Also, in terms of light gathering, the 70D's maximum ISO of 25600 blows away the paltry ISO 1600 maximum on an unmodified XTi. The 70D is actually fairly clean at ISO levels higher than the XTi is even capable of providing at all.
Although it is true that, assuming perfect detection of every single photon, sensor image quality is inherently limited by the sensor size, in reality, the rate of detection has improved dramatically over the past decade, the electrical noise floor of various electronic bits has improved, and the ability of chips to dissipate heat (which would otherwise cause thermal noise) has improved as well. All of these things have a significant impact on image quality, too. There's certainly reason to believe that we're approaching a limit beyond which future advancements will get harder and harder, and that the amount of improvement in each successive generation of sensor is likely to get smaller and smaller. That means that unless you care about the new body features (e.g. Wi-Fi), you're going to need to wait a greater number of generations to get a significant benefit out of a crop-to-crop upgrade. That doesn't mean that there won't come a time when an upgrade will be valuable, however; we're not yet to the point of having perfect detection of every photon.
And even if we don't improve the percentage of photons detected at all, there would still be room for improvement at the crop body sensor size. As plenoptic sensors become more capable, we might see those show up in crop body cameras. Imagine being able to correct focus errors just as easily as we correct exposure errors today.
Also, cameras are more than just a sensor. The other hardware limits what you can usefully do with the camera as well. For example:
- If you have an old enough crop body, you don't have video capabilities.
- The 70D adds Wi-Fi.
- The 70D has significantly better focusing capability than most (all?) existing crop bodies.
Heck, we might even see GPS in some future crop body, and for many people, that would be a feature worth paying for.
That's why sweeping generalizations like yours are harmful. They often mislead people who don't know any better into thinking that a camera is nothing more than a sensor, and that sensor technology has no more room to improve. Both of those two beliefs are very, very wrong.
Now if you had said that there's little reason to upgrade from a 60D to a 70D, most folks would agree with you. However, your claim implies that there's little reason to upgrade from a 300D (6.3 MP, max ISO 1600, no video, no Wi-Fi, 7 AF points, no cross points, f/5.6 minimum autofocus aperture) to a 70D (5 MP, max ISO 25600, video, Wi-Fi, 19 AF points, 19 cross points, f/11 minimum autofocus aperture in live view mode), which is just plain laughable.
