If screen size is the thing you're worried about being future-proof, then that's not something to worry about because it's completely your own preference and you said you are comfortable with the size. As far as the rest of the phone, the specs are generally good and I would expect you would have no problem using this phone for some time.
However, there will certainly be innovations and changes within the next 3 years so you may find yourself wanting some of the new features that will inevitably come out. This is true of any phone you could get at any time. There is also the issue of software updates. Most phones are not supported for 3 years. I don't personally know how long Asus usually supports phones with upgrades. Typically manufacturers support phones for 18-24 months from release. So if you have the phone for 3 years, you may not be able to upgrade after that unless you root and load a different ROM, which may or may not still be being developed either that long after release.
If you get the 551ML model with 4GB RAM that's ahead of the curve right now so others will have to catch up at the moment. And choose the faster processor since there are two options and the 64GB storage size.
Now I'm not sure if the 64GB SD card max is a definitive permanent limit or not. I'm not sure why it doesn't support 128GB but we'll see soon if it recognizes one (my 128GB card got damaged so I only have 64 and 32 sizes now, but someone else on here said they are going to try their 128 card). It's possible you may need more storage space in the future as app sizes are getting larger and larger and it depends on what you store on your phone - like video or large files. But many people never even use 32 GB so it depends on your own personal usage whether this even matters to you. You can get 64GB internal storage plus 64GB SD card and have 128GB total - minus system size. For the average person that's plenty, but for some that may not be future-proof.
There's also the battery to consider. Battery life will certainly decline over that period of time, so it matters how dependent you are on long battery life on a normal basis. If you need it to last all day off charger day after day and expect you will still have this need 2-3 years from now, you may towards the end of that period be getting far shorter battery life than you started with.
Three years is pushing it with any phone as far as being up to date with current tech - hardware and software. If you're leaning more towards the two year side, I wouldn't be worried unless you're the type of person who is going to miss not having the latest and greatest.