It seems like there are a couple of aspects to this topic:
1. The "when" (will you replace your S)
Looking back, and on average, I own my cars for about 6-7 years before I sell them. Upon further reflection, there's a correlation with how many miles I typically drive per year and wanting to sell a car before it reaches/greatly exceeds 100K miles. Right or wrong, this has been my MO and what I'm comfortable with. I suspect that there's a possibility that if I drive less in the future, I may be more comfortable keeping a car longer. But as things stand, I would anticipate that I will stay true to form w/r/t the ownership period stated earlier.
That said, for my S specifically, I plan on doing a re-evaluation in 3 years time (I've owned it for almost a year now) in case the technology evolves so rapidly as to effectively make my S pretty outdated. If this happens, I suspect my S at that point (and at the right price point) will still be attractive to some people (perhaps first time future Tesla owners), but I plan on staying on top of the latest game-changing tech as much as possible.
2. The "what" (will the replacement be)
As mentioned above, I've owned my S75D for almost a year now and I still have a grin in my face every morning as I see and walk up to it in my garage before driving it to work (when people would ask me why would I pay so much for a car, I would in turn list all the benefits but also ask them, how do you quantify happiness?). I want and hope my future cars to be Teslas as well. I'm a shareholder and cheering hard for them every day to succeed. I think they have a huge head start and are well positioned to continue to be the leader in BEVs, but they have to execute and do so smartly. In about 2-3 years, the competition is going to get super intense, which is why Telsa needs to be even more on top of their game.
Among any planned cars from any other manufacturers, the only one that has even come close to be drawing my attention is the Porsche Taycan. But the reason why I love my S so much is the combination of how it drives, the whole tech, the minimalist yet modern dash and interior, ease of traveling due to extensive SuperChargers coverage, and last but not least, the overall mission of the company. Tesla has set such a high bar in my mind that for me to consider any other car company, they will have to clearly beat it in at least 4 of those 5 things.