As far as non-squishy mages go, yes, they are entirely possible. That's the advantage of classless systems such as SR : roles aren't as clearly defined and you can hybridize character concepts in a way that's qualitatively different from multiclassing.
However, this will still make you worse at your primary role most of the time. Mages are scarce on BP to begin with- raising your Magic stat, buying appropriate qualities, half a ton of specific skills, spells, pushing otherwise rarely used attributes for soaking drain, that adds up. Simultaneously, the mage still has to buy a whole bunch of essential mundane skills.
If you'd ever tried to build a mage, you'd be aware of this, it's a pain in the ass to make them at least survivable in situations where the sam would shine.
Now, there's obviously buffs for everything- but unlike the boosts of a sam, these are temporary, take time to set up, require expensive ressources to be maintained comfortably and might be missing at the most inopportune moment. Having to move through a single ward undetected may shut off your entire defenses. Stepping into a BGC can leave you completely defenseless, or at least significantly impair you.
From one moment to the next, your additional IP and magical armor are gone. At best, you have to cast them all over again. At worst, you are stuck in an area where you are effectively a very poorly built 300 BP character who has to hope he doesn't accidentally shoot himself in the foot.
Sprawls aren't littered with zones that shut your bioware off.
A sam can also shoot opponents he can't see. The -6 for invisible targets shouldn't stop a good shooter, and he can still use ultrasound or radar, which doesn't work for spellcasting. The mage can't do that.
Just toss a thermal smoke grenade at the mage and use your ultrasound goggles to target him. He'll love that.
Yes, there's ritual magic. That just takes half a day and additional preparation. There's a reason why so many players don't even bother with it, in spite of the drastic advantages it would offer.
On top of that, SHOOT THE MAGE FIRST has been drilled into every corpsec squad ever. As soon as they realize that you're the caster, you're hosed. Better hope that your troll tank can keep them from pumping you full of tungsten alloys.
There's good reasons for mundanes to be afraid of mages, but mages themselves don't have it any better in this regard, they will get into situations where they feel utterly defenseless in comparison to other roles. I hate walking into a fight whenever i'm playing a caster. Staying in the background busy with summoning and mind control is so much easier and doesn't make me feel like a sitting duck.