Eventually it will be a bit difficult to avoid the altered genes.
Well, kind of. All genes do that in an outcrossing species (a crop that pollinates others readily, like corn or squash). In a natural population, selection pressure will influence the spread of the gene throughout the population, however, crops are not a natural population. For example, I have seed of blue, red, white, and yellow corn, and seed of all sorts of heirloom squash (orange and lumpy, bright red and smooth, pale and long). How is it possible that each of those still manages to exist, if genes inevitably spread throughout the population? Simple, controlled crossing. Genes inserted though biotechnology are no different. If you are preserving a population of, say, heirloom crops, you don't want any crossing anyway, and if you are simply buying hybrid seed every year, which many farmers do, it doesn't matter what they get crossed with.
but to have plants that resist Roundup get big doses of Roundup to kill other plants.
That is a misconception. They do not have to withstand 'big doses' of the herbicide; do you honestly think that farmers are spending extra money on seed so they can spend extra money on herbicide? The gene inserted is an alternate form of an enzyme found in all plants; the amount you need to spray is not a 'big dose' but rather enough to kill the weed.
That's arguably an irresponsible use of GMO.
Fair enough, I suppose you have a better method of weed control then? Your options, realistically, are tillage (very damaging to soil health), hand weeding (completely unfeasible), or harsher herbicides. Not good options, but that's what we've got, and if you're going to criticize crops resistant to glyphosate, which is one of the better herbicides out there, you are going to need a viable alternative; this is not a case of herbicide resistant crops versus nothing, it is a case of them or something else.
Certainly it makes Roundup a short lived herbicide, as plants develop resistance to it. And they will.
Well, yes, just like weeds developed resistance to other types of herbicides. This does not mean you don't use them, it means we need to use them better to mitigate resistance by using multiple modes of action instead of over-relying on one mode of action (EPSPS inhibitors in the case of Round-Up). Additionally, conventional breeding is also used to make herbicide resistant crops; are you going to criticize conventional breeding as well?