In a nutshell, these low costs will be lower than when there was a lower SegWit adoption rate.
Witness data takes up a significant percentage of the transaction space in each block. SegWit effectively displaces the witness data, allowing more of the blocksize to be dedicated to more transactions within the 1MB block size, effectively speeding up the nature of these transactions. Naturally, this also means that unconfirmed transactions won't have to spend so much time in the queue, as each block can now fit more transactions.
In the end, the fees are mainly dependent on the users who pay those fees, which can increase greatly if we ever have another repeat of the 2017/2018 Holiday FOMO run, but technically speaking, the fees should be lower than those of non-SegWit transactions because of how timely those transactions take place in each block compared to non-SW transactions.
Typical transactions (1 Input/2 Outputs) are about 226 bytes in size, while SegWit transactions are either:
- Nested P2SH Address: 166 bytes (26.5% discount)
- Bech32 Address: 141 bytes (37.6% discount)
Imagine this on a larger scale.
Given that the last block (517090 at the time of this post) consisted of 32% SegWit transactions
[1], that is nearly a third of the entire block size that is hosting transactions with these witness discounts.
During the FOMO period between December 2017 and January 2018, the SegWit adoption rate averaged under 13%. With a difference of nearly 20% more current SegWit adoption, I'm going to go out on a limb here and state that if we encounter another FOMO bull market run, the average transaction fee will be at least 20% lower than what it was during DEC/JAN, possibly more, due to the nature of how many more transactions can fit within the 20% SegWit difference...
Either way, this is all speculation, and we won't really know how much of an impact SegWit truly has until we endure another run like the last one we've experienced, so take my findings and speculation with a grain of salt
[1] According to
http://segwit.party/charts/