Transactions are not guaranted to be validated I have read some posts about transactions never to be confirmed, but those post implied small transfers with no fees.

In my case my two transactions have something like 0.000428 BTC of fee.

I don't know if the fee is too small. I have not chosen, it was my wallet Coinmoni that did it for me.

Anyway after almost 3 days without the 2 transactions being confirmed... my first impression is that there is a flaw in the Bitcoin system. What is the guarantee the transaction ever happens ? 

No guarantee. Am I wrong ? It's up to miners to choose the block they will compute.

What's the community thinking about transfers not being validated ? Isn't it a problem if Bitcoin really wants to become a public payment method, if transfer/transactions are not guaranted to be validated ?
When you broadcast a transaction, to get a confirmation you're bidding for blockchain space, a limited good. Since blocks currently supply only one megabyte of space, miners maximize their profit by selecting transaction in order of fee rate, i.e. the number of satoshis offered to the miners per byte of transaction size. (Unless there is an additional out-of-band payment that skews selection.) The amount of value transferred has no direct influence on the selection, and there is no guarantee for inclusion, the transaction selection is the miner's prerogative.

It is however your prerogative to increase your bid by updating and re-broadcasting your transaction.

Yes, this is a problem to some degree which is why there are protocol upgrades (e.g. SegWit, Schnorr Signatures, Signature Aggregation, Lightning Network)  waiting to be activated (or in the making) that will provide a larger throughput.