It prevents spam to a certain extent. In most cases an entity could still spam blocks with low chances of confirming transactions, but even if we know it's a spam transaction, it on the other hand isn't spam at all.

As long as transactions contain a fee, even if it's just a low one, it's still a valid transaction that can't be marked as spam, while in reality it might very well have that intention behind it.

According to you, an important transaction as per the economical sense, is a transaction containing a higher than average fee, with higher overall chances of getting next block confirmations, which isn't always so.

Higher fees don't always translate into 'more important' transactions. It's just how the economy goes.

So are you thinking of bitcoin as a currency or a commodity?  Because if you want it to be a currency, you need to be able to use it for purchases.  If the transaction fee is higher than the thing you want to purchase, then why are you going to use BTC?  Try buying $10 worth of coffee.  To get a transaction to occur in a timely matter, you need a $40 transaction fee.  Does that make sense?  Hell no.  We are moving further away from thinking of bitcoin as a currency and that is counterproductive to further adoption.  Try explaining to a new adopter that you have a great new currency that we want you to be able to use anywhere, any time.  Then tell them if you want to buy something all you have to do is pay $40 in addition to the purchase price (today).

If the issue continues, it will only get worse.  Maybe by summer we are up to $100 transaction fees.  So when you say "the fee is not that big" you have to ask in what way.  Because if you are only thinking of bitcoin as a commodity then maybe you are right (at least with people who are transfering large amounts of bitcoin).  But if you want it to be a currency which you want to use on a daily basis, then it is a huge issue.

So wouldn't a basic solution be;

1) Set transaction fees at a static (acceptable) rate.
2) Load transactions to a block based on the order in which they were placed in the blockchain.

There may be times of congestion, but at least you know that all transactions would go through in timely manner.

This would still allow large mining facilities to be able to collect their own fees, but they couldn't really manipulate it by increasing the fees themselves.

yea, people are facing problem due to high transaction fee and slow transaction while doing transaction but this is only due to high traffic on the network of blockchain and we hope that this issue will be solve soon as they will working on this issue because they don't want to dishurt their customers.

So
1)  set it to the rate it used to be when it was created.  No effect on decentralization.

2)  There are not that many transactions occurring because if that were true, everyone would be affected by that today including those paying the highest transaction fees.  Maybe it delays a transaction by a few hours, not days/weeks/months as paying a lower tx fee currently does.