Well, yes and no.
I agree that people should learn to live with their products longer then then 12 months cycle companies like Apple want to promote. Your 1 year old iPhone is not obsolete a year later regardless of Apple's aggressive marketing rhetoric. If companies were to adopt a 2 year cycle, it would have a significant impact on reducing greenhouse emissions as you are not having to produce phones every year to upgrade a large percentage of their consumer base.
However, making a phone modular and upgradable will be a horrible product. It would be much thicker then what we have today, be full of screw holes and panels, and then you would have to get the whole industry to agree to some kind of standards so that battery, RAM, storage, camera or CPU components or even screens would be swapped out and replaced. Otherwise you create a market of tones of proprietary after market parts, the sum of manufacturing ALL these parts does not mean a total reduction of greenhouse emissions.
Greenpeace should push for longer cycles between new product releases to reduce manufacturing impact, period. But creating a product that can be full of Frankenstein parts and a whole new industry of aftermarket manufactured components is not idea for anybody. Ultimately it comes to reducing manufacturing, period, but reducing phone manufacturing in place of numerous after market parts manufacturing is a placebo for what impact it will have aon reducing greenhouse emissions.
In the meantime, companies like Apple that take responsibility to accept their old products, and reclaim and recycle components, at least mitigates some emissions caused by making a new generation of product.