When you purchase your seat on a plane, your price includes the space that comes from its width and its incline. The physical space of your seat includes what is between the armrests and those precious few inches behind it when in the reclined position. It is up to the person in possession of the seat how to use or when to use its incline and that available space. If you want or need more space, there is the option of purchasing it, by choosing a different seating category.
If the person in front of you inclines their seat to access their purchased space, your option is to get that space back, is by also inclining your own seat to access your own purchased space. If you do not recline, then your physical space is diminished by the incline of the seat in front of you. If someone has a seat that does not incline, then oftentimes that seat category is less expensive and they made the informed choice to purchase it based on economics and not personal comfort.
If you choose to not purchase seats with more room at higher fares, then you must accept that you do not have control of both your space and the person in front of you. This is exceptionally confrontational and exhibits passive aggressive behaviour towards another passenger. It is completely unacceptable to prevent someone from using the space they have purchased, simply because you feel entitled to your space plus theirs.
Unfortunately, most airlines have configured their planes to maximize profitability for the economy class passenger by making the available space much smaller than it used to be. A diligent traveler can access various airline plane configurations to determine which ones offer more room in economy seating and choose the airline they fly with based on that or they can simply pay the premium for the space they require, on any particular flight.
Courtesy, kindness and communication with and towards your fellow travelers make for a much more pleasant experience when flying in the tight spaces of today’s economy class. But it does not mean you have to give up your own purchased space to accommodate someone else, when they had the option to choose different seating at a higher price, if they needed the extra few inches for the duration of the flight.
The etiquette for Economy Class seating while flying is to be aware of the person behind you and beside you and to take up no more space than you purchased for your personal comfort levels while in transit. If you need more space, then it is up to you to buy the appropriate sized seat to accommodate it and to not prevent someone else from using theirs.