Is replacing the potting soil really necessary as long as the roots don't need to be trimmed or anything like that? - is similar to asking if you have to drink as long as your body doesn't need water. It's inevitable that it will, just as it's inevitable that root congestion and soil collapse (of highly organic soils like Miracle Gro, the 5:1:1 mix, et al) are assured. To give your plants the best opportunity to grow as near their genetic potential as possible, you need to be proactive in ensuring your soil is structurally able to do its job, that roots aren't overly congested. Think of it in terms of preventive maintenance ..... like the mechanic on the old Framm (auto parts) commercial says with a slightly evil grin, "You can pay me now, or you can pay me later."

Mineral soils (gardens) don't compact if the particles are large enough, but we know that silt and clay soils DO compact. If we can get enough organic matter into the soils and ensure good drainage, soil biota will be active, moving through the soil and improving tilth - keeping it from compacting. Containers are generally inhospitable environs for soil life, so as particles become increasingly smaller as they break down, compaction and water retention increase, robbing roots of the O2 they need to remain healthy & function normally.

Soils like the gritty mix, because they are 2/3 inorganic remain serviceable indefinitely, but root congestion marches on. Many growers allow used gritty mix to dry, then put it in a tub and 'float off' the organic fraction (bark) and reuse the Turface/granite. I don't bother because it often has weed seeds in it and isn't worth my effort (a personal decision). Also, many soils contain pathogens that could carry over to the next batch. There are other reasons as well, but I guess the point I'd impress is that root maintenance of perennial plants is a good idea if your plan is to keep a plant for the long term and keep it healthy. Regular root work and pruning of the top has a rejuvenating effect on plants, too.

In the end, it's up to you. Some people are happy buying new plants and tossing the old as vitality wanes from neglecting the maintenance. I make no judgement about others who prefer that to regular maintenance - it's simply how they order their priorities, and I'm fine with whatever they decide - my only goal is to make sure people understand what's involved in making the decision. For myself, I look at it as a personal failure if a plant dies while in my care. I hate it. ;-)

I think that just the understanding of what limitations are associated with things like soil collapse and root congestion go a long way toward making most of us want to guard against them to whatever degree we deem appropriate. This is a case where no one should argue that someone must do this or that, or pass judgment if they don't, but at the same time it shouldn't be said that these things have no impact on growth/vitality just because someone chooses to ignore them.

Best luck!!

Al

You can supplement food via chemicals or mixing compost into the top layer so why do you need to change the potting soil for this reason

You may need to for the opposite reason--not to replenish nutrients, but to flush out accumulated salts/fert. For many organic potting media, they aren't usually very flushable before they break down, and after they start to decompose more, flushing would tend to remove a lot of the soil as well.

tapla's answer is inclusive and I wish I could remember it word for word. This question comes up so often. I once worked for a propagating nursery that kept all their mother stock in 15 or 20 gallon containers, and repotting in new soil was a regular chore needed to keep the stock growing and available for cuttings. Al