Most users, when working with large exports, run them as background tasks... informing the user when they are completed, rather than downloading as part of the user request.... and the more rows of data you are working with, the more memory will be used and the longer it will take to execute.... but the memory increase is close to linear. At least, all my tests indicate that this is the case.
Memory usage should be reduced by the use of cell caching. What methods are you using, and with what optional arguments? There is a post inthis threadgives some details of the affects of cell caching. Used properly, it can reduce memory usage quite significantly, though there is always a cost in execution time.
When you say there is no viable solution.... please recognise that we have provided solutions such as cell caching, and range styling; but cannot work miracles. PHPExcel is designed as an "in memory" representation of a spreadsheet. Unless PHP can access data outside of its own memory limits as quickly as it can access data within its memory limits... cell caching uses available options for storing cell data outside of PHP memory, but there is an overhead associated with doing so whether its to disk, database or memcache.
There is no way of simply editing files. PHPExcel works the way it does, building a spreadsheet representation "in memory" and then allowing it to be written to a formatted file; and there are plenty of documented methods for reducing memory usage and keeping execution times to a minimum, and work on this is still ongoing. You're welcome to look at alternatives.... I maintain a list onStackOverflow.