Because of AI model’s limitations, I am unable to provide a single 15000-word article. However, here’s a 900-word snippet on the topic:

Indoor succulents have a reputation for resilience and versatility making them a favorite among plants enthusiasts. One commonly asked question is, “How long do indoor succulents live?” The simple answer is that these plants can live from anywhere between 2-25 years, depending on factors such as species, care, and environment.

A closer look at Species:
There are hundreds of succulent species, and their lifespan varies considerably. Jade plants (Crassula ovata), for example, are notorious for their longevity, with proper care, they can live to be 70-100 years old. Sempervivum, also known as ‘hen and chicks,’ tends to have a life-cycle of about 3-4 years. While Agave succulents are known as “century plants” because of their lengthy growth period, yet most only live between 10-30 years.

The importance of Proper Care:
The lifespan of an indoor succulent is greatly influenced by how it’s cared for. Succulents thrive in conditions that mimic their native, harsh environments. They require plenty of sunlight, so keeping them in a spot that gets a good 6 hours of sunlight is essential. Most indoor succulents are thwarted by poor lighting conditions, stunting their growth and thus reducing lifespan.

Then there’s the question of watering – superbly paramount to succulent care. Most succulents thrive in dry conditions, with watering needs incredibly minimal. Overwatering is a common misstep which often leads to root rot, a dire condition that could bring the lifespan of your succulent to an abrupt end.

For indoor succulents, it’s advisable to use the “soak and dry” method of watering. This involves letting the soil dry out completely before watering profusely, simulating a heavy rainfall followed by a drought. This watering technique boosts the plant’s health, helping it live longer.

A consideration of Environment:
Environment implies two things – climate and soil. Succulents love warm and dry climates, so if you live in a region with a cold and wet climate, you might find it a bit challenging to prolong the life of your succulent.

Temperature plays a significant role too. Indoor succulents prefer a temperature range of 70-80 degrees Fahrenheit during the growth season (Spring and Summer), and 50-60 degrees during their dormant season (Fall and Winter).

The type of soil your succulent resides in is just as important. Succulents like well-draining soil mix that doesn’t retain water for a long period. Excess water in the soil can lead to root rot, drastically reducing the plant’s lifespan. A mixture of potting soil, coarse sand, and pumice or perlite is recommended for most succulents.

Where exactly you place your indoor succulent also constitutes the environment. Not all locations in your house will provide the requisite 6 hours of sunlight needed by these plant types. A south-facing window, for example, offers the best light. On the flip side, if such a spot isn’t available, you can supplement with a grow light, another effective technique to maximize your succulent’s lifespan.

Role of propagation:
Propagation is another factor that determines how long indoor succulents live. Succulents can reproduce through leaf and stem cuttings, offsets, or seed collection, and an understanding of this aspect is key to prolonging your succulents’ longevity.

Consider a succulent like Sempervivum; it’s monocarpic, meaning it dies after flowering. However, before reaching this stage, it produces many offsets, and by harvesting and planting these offsets, you carry on its lineage essentially retaining the ‘life’ of the original plant.

Potential threats and how to troubleshoot them:
Despite their inherent hardiness, indoor succulents are susceptible to a host of threats. These include pests like aphids, mealybugs, spider mites, and fungus gnats. Pest infestations can impact your succulent’s health, slowing growth, or in severe cases, causing the plant to die. Regularly inspect plants for signs of pests and treat promptly with a suitable insecticide or homemade solution.

Everyone wants their indoor succulents to reach their maximum lifespan, but hectic schedules, lack of knowledge, or general forgetfulness can mean things go awry. It’s crucial to identify common signs of declining succulents’ health.

Wilting, discoloration, softness, or dropping leaves are signs your succulent isn’t as healthy as it should be. Understanding these signs and knowing how to react could mean the difference between a prolonged life and premature death for your succulent.

Conclusion:
In summary, with the right care, perfect environment, adequate propogation, and prompt pest control, your indoor succulent can have an extended lifespan. Just like any houseplant, the longevity of succulents is largely dependent on how well they are cared for and the conditions they live in. With the right mix of all the above, you can enjoy the beauty and benefits these unique plants provide for many years.

In places where it’s almost impossible to grow outdoor plants, having long-lived indoor succulents adds a splash of refreshment. Indeed, the effort you put into caring for your indoor succulent today will pay off in joyous greenery for many years to come.

Remember, irrespective of the longevity of the species you pick, the care you provide to your succulent will always be the pivotal determinant of how long it will thrive in the cozy corners of your dwelling.