When we exchange your valuable Bitcoin or Ethereum for a different alt coin,
Ethereum itself is an altcoin itself and using bitcoin and ethereum in the same context doesn't make much sense! One is decentralized, secure and does what it was supposed to do and the other is centralized, mutable, buggy and doesn't do what it was promised to do.
Ethereum offers more fast and cheap transactions
No it doesn't.
Actually under same circumstance (same level of usage) ethereum transactions are far more expensive than bitcoin's. We saw that during the 2017 ether pump due to the ICO craze where it became nearly impossible to use due to high fees.
Ethereum also has slower transactions because as it was pointed out 1 confirmation on the mutable ethereum blockchain is not the same as 1 confirmation on immutable bitcoin blockchain.
When compared between Bitcoin and Ethereum, in general Ethereum’s transactions are faster and cheaper. It is a very important point when evaluating a coin.
It is more important to first understand what "fast" means.
Fast transaction doesn't mean when they get included in a block, it means when they become irreversible.
One confirmation on Bitcoin blockchain is usually equal to dozens up to in certain cases thousands of confirmation on other chains, and that's assuming they too have immutable blockchains. For an altcoin with a mutable blockchain where they have a history of rolling back blocks, no number of confirmation can be considered truly safe anymore.
it can happen for Ethereum to be more expensive at times, compared to Bitcoin.
No, that's impossible simply because Ethereum is useless (it's only used for hyped up short lived ICO garbage and its family) and also most importantly ether has an unlimited supply so like fiat it will continue losing value in long run not gain value.
SegWit transactions are simply smaller in size compared to legacy transactions because all witness data (signatures) is separated from transaction data.
SegWit transactions have a smaller weight not size.
The witness data is also NOT separated from the transaction data. It is still part and parcel of the transaction.
a limited number of transactions can only be fitted into each Bitcoin block,
All blockchain based cryptocurrencies have a block size cap so they can only include a finite number of transactions in each block.
By separating all witness data, SegWit effectively decreases size limits for each transaction and effectively increases Bitcoin’s block size limit,
As I said witness is not separated. The block and transaction structure is changed so that transactions can contain more data and blocks to contain more transactions in a backward compatible way.
A large pre-mine is making a project centralized
And ethereum has a massive premine ...
Still, people are buying coins from unreliable coders like Tron from Justin Sun or even weaker DeFi coins like Acala Stablecoin from Polkadot’s supplier Web 3.
Unfortunately, some people will only learn after a loss.
Well, such people aren't actually buying "coins", they are making bets on random stuff trying to make some money gambling. They might as well be in a race track betting on running horses or something like that!
It is big controversy for a long time, if a coin makes PoW or PoS, like it was for Ethereum [ETH]. Not for Bitcoin because Bitcoin developers have decided PoW is best because of security.
It's not exactly a controversy, it is a topic that has generated a lot of FUD.
And the choice of PoW is not because it is "best", but because there is no alternative. For example PoS is just fundamentally flawed so it is not even worth considering since it was proven at least a decade ago that it is not a viable algorithm.