If you don't ride two up, pull the rear seat off and remove the riser for the rack, bolting the plate directly to the bike. Less moment arm vibration and less chance of future cracking.
Odds are good AltRider will replace it for free or minimal cost. Failing that, yes, you could have it welded. A good welder would reinforce the areas behind the cracks with some additional flat stock, welded to the main piece.
Any weight on the rack that hangs off the back will essentially act like a lever, flexing the rack back and forth as you ride. Aluminum fracture rates are distinct. There is no if, only when it will crack. Some designs place that when a long way out, others not so much.
Altrider said they couldn't get just the frame. Offered me 35% off a new one. That would be $149 plus shipping. Sh-t. Going to the welder. Kinda ridiculous I can't get the frame for this PREMIUM ADV ACCESSORIES.
Sorry to hear and see those failure issues. Maybe you had loose or over torqued bolts? Some of the complaints in the past centered on the beveled bolt holes being too thin. The side cracks are disconcerting though. It could be a weight distribution or tie down issue. I have had mine on the bike for 5 years with no issues. It's hard to believe I have had the bike that long. Lots of trips, on road & off, loaded for COBDR and Alaska (solo). Lots of trail riding. I either have my Nelson-Rigg 30L tail bag on (most of the time) or my 45L duffel. For both of the above trips the duffel probably weighed somewhere 40 pounds, but was half on the seat & half on the rack. Pretty much the same width of the big top plate of the rack. The tail bag is almost the same size as the rack too. The tail bag loaded might be about 10 pounds at most. It's nylon with just rain gear & gloves and maybe a jacket or 2 in it.