It's hard to diagnose the cause of plant loss without images or a description of the symptoms (before they were fully brown, wilted, or dead) or information about what kinds of plants they were, since they don't all share the same vulnerabilities. Soil moisture is usually the culprit behind a weakening or direct dieback for indoor plants. This could be a pot getting too dry between waterings or staying too wet; it's hard to tell without more information, and the symptoms of both can overlap. Did the pots have drainage? If not, over-watering is more common, and can take place when the lower roots essentially drown in the soil because of too little oxygen. (Roots do need to "breathe" a bit.)

If the potting soil is several years old, it would be best to replace it, not so much because of a risk of transmittable pests or diseases (though it's not zero risk), but because old potting soil degrades over time, not draining as well and accumulating mineral residues that can stress or injure roots.

The time of year for replacing houseplants doesn't matter, as long as you can get any purchased plants home without baking or chilling them in the car in the process. Some houseplants are more heat-sensitive, some are more chill-sensitive, so it depends on the species. Not using A/C in the house might have stressed some plants, but many of those grown indoors are tropical in nature and probably would be more bothered by no heat in winter than no A/C in summer. If they were sitting in a sunny window, though, that heat build-up might have been enough to cause some dieback.

Local garden centers tend to devote more greenhouse space to houseplants in winter, since they don't use that area for stocking herbs, annuals, vegetables, or other plants. (Some may also sell herbs indoors for people to grow inside for the winter, but it's not as major a crop for them then as it is in spring and summer.) Therefore, the selection might be best in winter, though that can depend on where you shop and how they prefer to stock the store based on what other plants they sell.

Some houseplants are tougher than others, tolerating conditions that would be too stressful for some species. While no houseplant truly thrives in low light, since indoors that is much dimmer light than even full shade outside, there are some plants in that group that do well for years as long as they're not over-watered. Snake plant (Sansevieria) and cast-iron plant (Aspidistra) are two examples. They grow very slowly, but also put up with a lot. For bright light areas, many succulents do well (and also grow somewhat slowly), and in intermediate conditions, Boston fern and its relatives are also fairly forgiving. On the other hand, species harder to keep thriving include palms, Maidenhair ferns, bird-of-paradise, and rex begonias.

Miri